Sean Quinlan and Sean Dorward
 Bell Labs, Lucent Technologies
 
 Abstract
 
 This paper describes a network storage system, called Venti, intended
 for archival data. In this system, a unique hash of a block's contents
 acts as the block identifier for read and write operations. This
 approach enforces a write-once policy, preventing accidental or
 malicious destruction of data. In addition, duplicate copies of a
 block can be coalesced, reducing the consumption of storage and
 simplifying the implementation of clients. Venti is a building block
 for constructing a variety of storage applications such as logical
 backup, physical backup, and snapshot file systems.
 
 We have built a prototype of the system and present some preliminary
 performance results. The system uses magnetic disks as the storage
 technology, resulting in an access time for archival data that is
 comparable to non-archival data. The feasibility of the write-once
 model for storage is demonstrated using data from over a decade's use
 of two Plan 9 file systems.
 
 1. Introduction
 
 Archival storage is a second class citizen. Many computer environments
 provide access to a few recent versions of the information stored in
 file systems and databases, though this access can be tedious and may
 require the assistance of a system administrator. Less common is the
 ability for a user to examine data from last month or last year or
 last decade. Such a feature may not be needed frequently, but when it
 is needed it is often crucial.
 
 The growth in capacity of storage technologies exceeds the ability of
 many users to generate data, making it practical to archive data in
 perpetuity. Plan 9, the computing environment that the authors use,
 includes a file system that stores archival data to an optical jukebox
 [16, 17]. Ken Thompson observed that, for our usage patterns, the
 capacity of the jukebox could be considered infinite. In the time it
 took for us to fill the jukebox, the improvement in technology would
 allow us to upgrade to a new jukebox with twice the capacity.
 
 Abundant storage suggests that an archival system impose a write-once
 policy. Such a policy prohibits either a user or administrator from
 deleting or modifying data once it is stored. This approach greatly
 reduces the opportunities for accidental or malicious data loss and
 simplifies the system's implementation.
 
 Moreover, our experience with Plan 9 is that a write-once policy
 changes the way one views storage. Obviously, some data is temporary,
 derivative, or so large that it is either undesirable or impractical
 to retain forever and should not be archived. However, once it is
 decided that the data is worth keeping, the resources needed to store
 the data have been consumed and cannot be reclaimed. This eliminates
 the task of periodically "cleaning up" and deciding whether the data
 is still worth keeping. More thought is required before storing the
 data to a write-once archive, but as the cost of storage continues to
 fall, this becomes an easy decision.
 
 This paper describes the design and implementation of an archival
 server, called Venti. The goal of Venti is to provide a write-once
 archival repository that can be shared by multiple client machines and
 applications. In addition, by using magnetic disks as the primary
 storage technology, the performance of the system approaches that of
 non-archival storage.
 
 2. Background
 
 A prevalent form of archival storage is the regular backup of data to
 magnetic tape [15]. A typical scenario is to provide backup as a
 central service for a number of client machines. Client software
 interfaces with a database or file system and determines what data to
 back up. The data is copied from the client to the tape device, often
 over a network, and a record of what was copied is stored in a catalog
 database.
 
 Restoring data from a tape backup system can be tedious and error
 prone. The backup system violates the access permission of the file
 system, requiring a system administrator or privileged software to
 perform the task. Since they are tedious, restore operations are
 infrequent and problems with the process may go undetected. Potential
 sources of error abound: tapes are mislabeled or reused or lost,
 drives wander out of alignment and cannot read their old tapes,
 technology becomes obsolete.
 
 For tape backup systems, a tradeoff exists between the performance of
 backup and restore operations [1]. A full backup simplifies the
 process of restoring data since all the data is copied to a continuous
 region on the tape media. For large file systems and databases,
 incremental backups are more efficient to generate, but such backups
 are not self-contained; the data for a restore operation is scattered
 across multiple incremental backups and perhaps multiple tapes. The
 conventional solution is to limit the extent of this scattering by
 performing a full backup followed by a small number of incremental
 backups.
 
 File systems such as Plan 9 [16, 17], WAFL [5], and AFS [7] provide a
 more unified approach to the backup problem by implementing a snapshot
 feature. A snapshot is a consistent read-only view of the file system
 at some point in the past. The snapshot retains the file system
 permissions and can be accessed with standard tools (ls, cat, cp,
 grep, diff) without special privileges or assistance from an
 administrator. In our experience, snapshots are a relied-upon and
 frequently-used resource because they are always available and easy to
 access.
 
 Snapshots avoid the tradeoff between full and incremental backups.
 Each snapshot is a complete file system tree, much like a full backup.
 The implementation, however, resembles an incremental backup because
 the snapshots and the active file system share any blocks that remain
 unmodified; a snapshot only requires additional storage for the blocks
 that have changed. To achieve reasonable performance, the device that
 stores the snapshots must efficiently support random access, limiting
 the suitability of tape storage for this approach.
 
 In the WAFL and AFS systems, snapshots are ephemeral; only a small
 number of recent versions of the file system are retained. This policy
 is reasonable since the most recent versions of files are the most
 useful. For these systems, archival storage requires an additional
 mechanism such as tape backup.
 
 The philosophy of the Plan 9 file system is that random access storage
 is sufficiently cheap that it is feasible to retain snapshots
 permanently. The storage required to retain all daily snapshots of a
 file system is surprisingly modest; later in the paper we present
 statistics for two file servers that have been in use over the last 10
 years.
 
 Like Plan 9, the Elephant file system [18] retains many versions of
 data. This system allows a variety of storage reclamation policies
 that determine when a version of a file should be deleted. In
 particular,"landmark" versions of files are retained permanently and
 provide an archival record.
 
 3. The Venti Archival Server
 
 Venti is a block-level network storage system intended for archival
 data. The interface to the system is a simple protocol that enables
 client applications to read and write variable sized blocks of data.
 Venti itself does not provide the services of a file or backup system,
 but rather the backend archival storage for these types of
 applications.
 
 Venti identifies data blocks by a hash of their contents. By using a
 collision-resistant hash function with a sufficiently large output, it
 is possible to consider the hash of a data block as unique. Such a
 unique hash is called the fingerprint of a block and can be used as
 the address for read and write operations. This approach results in a
 storage system with a number of interesting properties.
 
 As blocks are addressed by the fingerprint of their contents, a block
 cannot be modified without changing its address; the behavior is
 intrinsically write-once. This property distinguishes Venti from most
 other storage systems, in which the address of a block and its
 contents are independent.
 
 Moreover, writes are idempotent. Multiple writes of the same data can
 be coalesced and do not require additional storage space. This
 property can greatly increase the effective storage capacity of the
 server since it does not rely on the behavior of client applications.
 For example, an incremental backup application may not be able to
 determine exactly which blocks have changed, resulting in unnecessary
 duplication of data. On Venti, such duplicate blocks will be discarded
 and only one copy of the data will be retained. In fact, replacing the
 incremental backup with a full backup will consume the same amount of
 storage. Even duplicate data from different applications and machines
 can be eliminated if the clients write the data using the same block
 size and alignment.
 
 The hash function can be viewed as generating a universal name space
 for data blocks. Without cooperating or coordinating, multiple clients
 can share this name space and share a Venti server. Moreover, the
 block level interface places few restrictions on the structures and
 format that clients use to store their data. In contrast, traditional
 backup and archival systems require more centralized control. For
 example, backup systems include some form of job scheduler to
 serialize access to tape devices and may only support a small number
 of predetermined data formats so that the catalog system can extract
 pertinent meta-data.
 
 Venti provides inherent integrity checking of data. When a block is
 retrieved, both the client and the server can compute the fingerprint
 of the data and compare it to the requested fingerprint. This
 operation allows the client to avoid errors from undetected data
 corruption and enables the server to identify when error recovery is
 necessary.
 
 Using the fingerprint of a block as its identity facilitates features
 such as replication, caching, and load balancing. Since the contents
 of a particular block are immutable, the problem of data coherency is
 greatly reduced; a cache or a mirror cannot contain a stale or out of
 date version of a block.
 
 3.1. Choice of Hash Function
 
 The design of Venti requires a hash function that generates a unique
 fingerprint for every data block that a client may want to store.
 Obviously, if the size of the fingerprint is smaller than the size of
 the data blocks, such a hash function cannot exist since there are
 fewer possible fingerprints than blocks. If the fingerprint is large
 enough and randomly distributed, this problem does not arise in
 practice. For a server of a given capacity, the likelihood that two
 different blocks will have the same hash value, also known as a
 collision, can be determined. If the probability of a collision is
 vanishingly small, we can be confident that each fingerprint is
 unique.
 
 It is desirable that Venti employ a cryptographic hash function. For
 such a function, it is computationally infeasible to find two distinct
 inputs that hash to the same value [10]. This property is important
 because it prevents a malicious client from intentionally creating
 blocks that violate the assumption that each block has a unique
 fingerprint. As an additional benefit, using a cryptographic hash
 function strengthens a client's integrity check, preventing a
 malicious server from fulfilling a read request with fraudulent data.
 If the fingerprint of the returned block matches the requested
 fingerprint, the client can be confident the server returned the
 original data.
 
 Venti uses the Sha1 hash function [13] developed by the US National
 Institute for Standards and Technology (NIST). Sha1 is a popular hash
 algorithm for many security systems and, to date, there are no known
 collisions. The output of Sha1 is a 160 bit (20 byte) hash value.
 Software implementations of Sha1 are relatively efficient; for
 example, a 700Mhz Pentium 3 can compute the Sha1 hash of 8 Kbyte data
 blocks in about 130 microseconds, a rate of 60 Mbytes per second.
 
 Are the 160 bit hash values generated by Sha1 large enough to ensure
 the fingerprint of every block is unique? Assuming random hash values
 with a uniform distribution, a collection of n different data blocks
 and a hash function that generates b bits, the probability p that
 there will be one or more collisions is bounded by the number of pairs
 of blocks multiplied by the probability that a given pair will
 collide, i.e.
 
 Today, a large storage system may contain a petabyte (10^15 bytes) of
 data. Consider an even larger system that contains an exabyte (10^18
 bytes) stored as 8 Kbyte blocks (~10^14 blocks). Using the Sha1 hash
 function, the probability of a collision is less than 10^-20. Such a
 scenario seems sufficiently unlikely that we ignore it and use the
 Sha1 hash as a unique identifier for a block. Obviously, as storage
 technology advances, it may become feasible to store much more than an
 exabyte, at which point it maybe necessary to move to a larger hash
 function. NIST has already proposed variants of Sha1 that produce 256,
 384, and 512 bit results [14]. For the immediate future, however, Sha1
 is a suitable choice for generating the fingerprint of a block.
 
 3.2. Choice of Storage Technology
 
 When the Plan 9 file system was designed in 1989, optical jukeboxes
 offered high capacity with respectable random access performance and
 thus were an obvious candidate for archival storage. The last decade,
 however, has seen the capacity of magnetic disks increase at a far
 faster rate than optical technologies [20]. Today, a disk array costs
 less than the equivalent capacity optical jukebox and occupies less
 physical space. Disk technology is even approaching tape in cost per
 bit.
 
 Magnetic disk storage is not as stable or permanent as optical media.
 Reliability can be improved with technology such as RAID, but unlike
 write-once optical disks, there is little protection from erasure due
 to failures of the storage server or RAID array firmware. This issue
 is discussed in Section 7.
 
 Using magnetic disks for Venti has the benefit of reducing the
 disparity in performance between conventional and archival storage.
 Operations that previously required data to be restored to magnetic
 disk can be accomplished directly from the archive. Similarly, the
 archive can contain the primary copy of often-accessed read-only data.
 In effect, archival data need not be further down the storage
 hierarchy; it is differentiated by the write-once policy of the
 server.
 
 4. Applications
 
 Venti is a building block on which to construct a variety of storage
 applications. Venti provides a large repository for data that can be
 shared by many clients, much as tape libraries are currently the
 foundation of many centralized backup systems. Applications need to
 accommodate the unique properties of Venti, which are different from
 traditional block level storage devices, but these properties enable a
 number of interesting features.
 
 Applications use the block level service provided by Venti to store
 more complex data structures. Data is divided into blocks and written
 to the server. To enable this data to be retrieved, the application
 must record the fingerprints of these blocks. One approach is to pack
 the fingerprints into additional blocks, called pointer blocks, that
 are also written to the server, a process that can be repeated
 recursively until a single fingerprint is obtained. This fingerprint
 represents the root of a tree of blocks and corresponds to a
 hierarchical hash of the original data.
 
 A simple data structure for storing a linear sequence of data blocks
 is shown in Figure 1. The data blocks are located via a fixed depth
 tree of pointer blocks which itself is addressed by a root
 fingerprint. Applications can use such a structure to store a single
 file or to mimic the behavior of a physical device such as a tape or a
 disk drive. The write-once nature of Venti does not allow such a tree
 to be modified, but new versions of the tree can be generated
 efficiently by storing the new or modified data blocks and reusing the
 unchanged sections of the tree as depicted in Figure 2.
 
 Figure 1. A tree structure for storing a linear sequence of blocks
 
 Figure 2. Build a new version of the tree.
 
 By mixing data and fingerprints in a block, more complex data
 structures can be constructed. For example, a structure for storing a
 file system may include three types of blocks: directory, pointer, and
 data. A directory block combines the meta information for a file and
 the fingerprint to a tree of data blocks containing the file's
 contents. The depth of the tree can be determined from the size of the
 file, assuming the pointer and data blocks have a fixed size. Other
 structures are obviously possible. Venti's block-level interface
 leaves the choice of format to client applications and different data
 structures can coexist on a single server.
 
 The following sections describes three applications that use Venti as
 an archival data repository: a user level archive utility called vac,
 a proposal for a physical level backup utility, and our preliminary
 work on a new version of the Plan 9 file system.
 
 4.1. Vac
 
 Vac is an application for storing a collection of files and
 directories as a single object, similar in functionality to the
 utilities tar and zip. With vac, the contents of the selected files
 are stored as a tree of blocks on a Venti server. The root fingerprint
 for this tree is written to a vac archive file specified by the user,
 which consists of an ASCII representation of the 20 byte root
 fingerprint plus a fixed header string, and is always 45 bytes long. A
 corresponding program, called unvac, enables the user to restore files
 from a vac archive. Naturally, unvac requires access to the Venti
 server that contains the actual data, but in most situations this is
 transparent. For a user, it appears that vac compresses any amount of
 data down to 45 bytes.
 
 An important attribute of vac is that it writes each file as a
 separate collection of Venti blocks, thus ensuring that duplicate
 copies of a file will be coalesced on the server. If multiple users
 vac the same data, only one copy will be stored on the server.
 Similarly, a user may repeatedly vac a directory over time and even if
 the contents of the directory change, the additional storage consumed
 on the server will be related to the extent of the changes rather than
 the total size of the contents. Since Venti coalesces data at the
 block level, even files that change may share many blocks with
 previous versions and thus require little space on the server; log and
 database files are good examples of this scenario.
 
 On many Unix systems, the dump utility is used to back up file
 systems. Dump has the ability to perform incremental backups of data;
 a user specifies a dump level, and only files that are new or have
 changed since the last dump at this level are written to the archive.
 To implement incremental backups, dump examines the modified time
 associated with each file, which is an efficient method of filtering
 out the unchanged files.
 
 Vac also implements an incremental option based on the file
 modification times. The user specifies an existing vac file and this
 archive is used to reduce the number of blocks written to the Venti
 server. For each file, vac examines the modified time in both the file
 system and the vac archive. If they are the same, vac copies the
 fingerprint for the file from the old archive into the new archive.
 Copying just the 20-byte fingerprint enables the new archive to
 include the entire file without reading the data from the file system
 nor writing the data across the network to the Venti server. In
 addition, unlike an incremental dump, the resulting archive will be
 identical to an archive generated without the incremental option; it
 is only a performance improvement. This means there is no need to have
 multiple levels of backups, some incremental, some full, and so
 restore operations are greatly simplified.
 
 A variant of the incremental option improves the backup of files
 without reference to modification times. As vac reads a file, it
 computes the fingerprint for each block. Concurrently, the pointer
 blocks of the old archive are examined to determine the fingerprint
 for the block at the same offset in the old version of the file. If
 the fingerprints are the same, the block does not need to be written
 to Venti. Instead, the fingerprint can simply be copied into the
 appropriate pointer block. This optimization reduces the number of
 writes to the Venti server, saving both network and disk bandwidth.
 Like the file level optimization above, the resulting vac file is no
 different from the one produced without this optimization. It does,
 however, require the data for the file to be read and is only
 effective if there are a significant number of unchanged blocks.
 
 4.2. Physical backup
 
 Utilities such as vac, tar, and dump archive data at the file or
 logical level: they walk the file hierarchy converting both data and
 meta-data into their own internal format. An alternative approach is
 block-level or physical backup, in which the disk blocks that make up
 the file system are directly copied without interpretation. Physical
 backup has a number of benefits including simplicity and potentially
 much higher throughput [8]. A physical backup utility for file systems
 that stores the resulting data on Venti appears attractive, though we
 have not yet implemented such an application.
 
 The simplest form of physical backup is to copy the raw contents of
 one or mores disk drives to Venti. The backup also includes a tree of
 pointer blocks, which enables access to the data blocks. Like vac, the
 end result is a single fingerprint representing the root of the tree;
 that fingerprint needs to be recorded outside of Venti.
 
 Coalescing duplicate blocks is the main advantage of making a physical
 backup to Venti rather than copying the data to another storage medium
 such as tape. Since file systems are inherently block based, we expect
 coalescing to be effective. Not only will backups of a file system
 over time share many unchanged blocks, but even file systems for
 different machines that are running the same operating system may have
 many blocks in common. As with vac, the user sees a full backup of the
 device, while retaining the storage space advantages of an incremental
 backup.
 
 One enhancement to physical backup is to copy only blocks that are
 actively in use in the file system. For most file system formats it is
 relatively easy to determine if a block is in use or free without
 walking the file system hierarchy. Free blocks generally contain the
 remnants of temporary files that were created and removed in the time
 between backups and it is advantageous not to store such blocks. This
 optimization requires that the backup format be able to represent
 missing blocks, which can easily be achieved on Venti by storing a
 null value for the appropriate entry in the pointer tree.
 
 The random access performance of Venti is sufficiently good that it is
 possible to use a physical backup without first restoring it to disk.
 With operating system support, it is feasible to directly mount a
 backup file system image from Venti. Access to this file system is
 read only, but it provides a natural method of restoring a subset of
 files. For situations where a full restore is required, it might be
 possible to do this restore in a lazy fashion, copying blocks from
 Venti to the file system as needed, instead of copying the entire
 contents of the file system before resuming normal operation.
 
 The time to perform a physical backup can be reduced using a variety
 of incremental techniques. Like vac, the backup utility can compute
 the fingerprint of each block and compare this fingerprint with the
 appropriate entry in the pointer tree of a previous backup. This
 optimization reduces the number of writes to the Venti server. If the
 file system provides information about which blocks have changed, as
 is the case with WAFL, the backup utility can avoid even reading the
 unchanged blocks. Again, a major advantage of using Venti is that the
 backup utility can implement these incremental techniques while still
 providing the user with a full backup. The backup utility writes the
 new blocks to the Venti server and constructs a pointer tree with the
 appropriate fingerprint for the unchanged blocks.
 
 4.3. Plan 9 File system
 
 When combined with a small amount of read/write storage, Venti can be
 used as the primary location for data rather than a place to store
 backups. A new version of the Plan 9 file system, which we are
 developing, exemplifies this approach.
 
 Previously, the Plan 9 file system was stored on a combination of
 magnetic disks and a write-once optical jukebox. The jukebox furnishes
 the permanent storage for the system, while the magnetic disks act as
 a cache for the jukebox. The cache provides faster file access and,
 more importantly, accumulates the changes to the file system during
 the period between snapshots. When a snapshot is taken, new or
 modified blocks are written from the disk cache to the jukebox.
 
 The disk cache can be smaller than the active file system, needing
 only to be big enough to contain the daily changes to the file system.
 However, accesses that miss the cache are significantly slower since
 changing platters in the jukebox takes several seconds. This
 performance penalty makes certain operations on old snapshots
 prohibitively expensive. Also, on the rare occasions when the disk
 cache has been reinitialized due to corruption, the file server spends
 several days filling the cache before performance returns to normal.
 
 The new version of the Plan 9 file system uses Venti instead of an
 optical jukebox as its storage device. Since the performance of Venti
 is comparable to disk, this substitution equalizes access both to the
 active and to the archival view of the file system. It also allows the
 disk cache to be quite small; the cache accumulates changes to the
 file system between snapshots, but does not speed file access.
 
 5. Implementation
 
 We have implemented a prototype of Venti. The implementation uses an
 append-only log of data blocks and an index that maps fingerprints to
 locations in this log. It also includes a number of features that
 improve robustness and performance. This section gives a brief
 overview of the implementation. Figure 3 shows a block diagram of the
 server.
 
 Figure 3. A block diagram of the Venti prototype.
 
 Since Venti is intended for archival storage, one goal of our
 prototype is robustness. The approach we have taken is to separate the
 storage of data blocks from the index used to locate a block. In
 particular, blocks are stored in an append-only log on a RAID array of
 disk drives. The simplicity of the append-only log structure
 eliminates many possible software errors that might cause data
 corruption and facilitates a variety of additional integrity
 strategies. A separate index structure allows a block to be
 efficiently located in the log; however, the index can be regenerated
 from the data log if required and thus does not have the same
 reliability constraints as the log itself.
 
 The structure of the data log is illustrated in Figure 4. To ease
 maintenance, the log is divided into self-contained sections called
 arenas. Each arena contains a large number of data blocks and is sized
 to facilitate operations such as copying to removable media. Within an
 arena is a section for data bocks that is filled in an append-only
 manner. In Venti, data blocks are variable sized, up to a current
 limit of 52 Kbytes, but since blocks are immutable they can be densely
 packed into an arena without fragmentation.
 
 Figure 4. The format of the data log.
 
 Each block is prefixed by a header that describes the contents of the
 block. The primary purpose of the header is to provide integrity
 checking during normal operation and to assist in data recovery. The
 header includes a magic number, the fingerprint and size of the block,
 the time when the block was first written, and identity of the user
 that wrote it. The header also includes a user-supplied type
 identifier, which is explained in Section 7. Note, only one copy of a
 given block is stored in the log, thus the user and wtime fields
 correspond to the first time the block was stored to the server.
 
 Before storing a block in the log, an attempt is made to compress its
 contents. The inclusion of data compression increases the effective
 capacity of the archive and is simple to add given the log structure.
 Obviously, some blocks are incompressible. The encoding field in the
 block header indicates whether the data was compressed and, if so, the
 algorithm used. The esize field indicates the size of the data after
 compression, enabling the location of the next block in the arena to
 be determined. The downside of using compression is the computational
 cost, typically resulting in a decrease in the rate that blocks can be
 stored and retrieved. Our prototype uses a custom Lempel-Ziv '77 [21]
 algorithm that is optimized for speed. Compression is not a
 performance bottleneck for our existing server. Future implementations
 may benefit from hardware solutions.
 
 In addition to a log of data blocks, an arena includes a header, a
 directory, and a trailer. The header identifies the arena. The
 directory contains a copy of the block header and offset for every
 block in the arena. By replicating the headers of all the blocks in
 one relatively small part of the arena, the server can rapidly check
 or rebuild the system's global block index. The directory also
 facilitates error recovery if part of the arena is destroyed or
 corrupted. The trailer summarizes the current state of the arena
 itself, including the number of blocks and the size of the log. Within
 the arena, the data log and the directory start at opposite ends and
 grow towards each other. When the arena is filled, it is marked as
 sealed, and a fingerprint is computed for the contents of the entire
 arena. Sealed arenas are never modified.
 
 The basic operation of Venti is to store and retrieve blocks based on
 their fingerprints. A fingerprint is 160 bits long, and the number of
 possible fingerprints far exceeds the number of blocks stored on a
 server. The disparity between the number of fingerprints and blocks
 means it is impractical to map the fingerprint directly to a location
 on a storage device. Instead, we use an index to locate a block within
 the log.
 
 We implement the index using a disk-resident hash table as illustrated
 in Figure 5. The index is divided into fixed-sized buckets, each of
 which is stored as a single disk block. Each bucket contains the index
 map for a small section of the fingerprint space. A hash function is
 used to map fingerprints to index buckets in a roughly uniform manner,
 and then the bucket is examined using binary search. If provisioned
 with sufficient buckets, the index hash table will be relatively empty
 and bucket overflows will be extremely rare. If a bucket does
 overflow, the extra entries are placed in an adjacent bucket. This
 structure is simple and efficient, requiring one disk access to locate
 a block in almost all cases.
 
 Figure 5. Format of the index.
 
 The need to go through an index is the main performance penalty for
 Venti compared to a conventional block storage device. Our prototype
 uses three techniques to increase the performance: caching, striping,
 and write buffering.
 
 The current implementation has two important caches of approximately
 equal size: a block cache and an index cache. A hit in the block cache
 returns the data for that fingerprint, bypassing the both the index
 lookup and access to the data log. Hits in the index cache eliminate
 only the index lookup, but the entries are much smaller and the hit
 rate correspondingly higher.
 
 Unfortunately, these caches do not speed the process of storing a new
 block to Venti. The server must check that the block is not a
 duplicate by examining the index. If the block is not contained on the
 server, it will obviously not be in any cache. Since the fingerprint
 of the block contains no internal structure, the location of a
 fingerprint in the index is essentially random. Furthermore, the
 archival nature of Venti means the entire index will not fit in memory
 because of the large number of blocks. Combining these factors means
 that the write performance of Venti will be limited to the random IO
 performance of the index disk, which for current technology is a few
 hundred accesses per second. By striping the index across multiple
 disks, however, we get a linear speedup. This requires a sufficient
 number of concurrent accesses, which we assure by buffering the writes
 before accessing the index.
 
 The prototype Venti server is implemented for the Plan 9 operating
 system in about 10,000 lines of C. The server runs on a dedicated dual
 550Mhz Pentium III processor system with 2 Gbyte of memory and is
 accessed over a 100Mbs Ethernet network. The data log is stored on a
 500 Gbyte MaxTronic IDE Raid 5 Array and the index resides on a string
 of 8 Seagate Cheetah 18XL 9 Gbyte SCSI drives.
 
 6. Performance
 
 Table 1 gives the preliminary performance results for read and write
 operations in a variety of situations. For comparison, we include the
 SCSI performance of the RAID array. Although the performance is still
 several times slower than directly accessing the disk, we believe the
 results are promising and will improve as the system matures.
 
 Table 1. The performance of read and write operations in Mbytes/s for
 8 Kbyte blocks.
 
 sequential reads
 random reads
 virgin writes
 duplicate writes
 uncached
 0.9
 0.4
 3.7
 5.6
 index cache
 4.2
 0.7
 -
 6.2
 block cache
 6.8
 -
 -
 6.5
 raw raid
 14.8
 1.0
 12.4
 12.4
 
 The uncached sequential read performance is particularly bad. The
 problem is that these sequential reads require a random read of the
 index. Without assistance from the client, the read operations are not
 overlapped and do not benefit from the striping of the index. One
 possible solution is a form of read-ahead. When reading a block from
 the data log, it is feasible to also read several following blocks.
 These extra blocks can be added to the caches without referencing the
 index. If blocks are read in the same order they were written to the
 log, the latency of uncached index lookups will be avoided. This
 strategy should work well for streaming data such as multimedia files.
 
 The basic assumption in Venti is that the growth in capacity of disks
 combined with the removal of duplicate blocks and compression of their
 contents enables a model in which it is not necessary to reclaim space
 by deleting archival data. To demonstrate why we believe this model is
 practical, we present some statistics derived from a decade's use of
 the Plan 9 file system.
 
 The computing environment in which we work includes two Plan 9 file
 servers named bootes and emelie. Bootes was our primary file
 repository from 1990 until 1997 at which point it was superseded by
 emelie. Over the life of these two file servers there have been 522
 user accounts of which between 50 and 100 were active at any given
 time. The file servers have hosted numerous development projects and
 also contain several large data sets including chess end games,
 astronomical data, satellite imagery, and multimedia files.
 
 Figure 6 depicts the size of the active file system as measured over
 time by du, the space consumed on the jukebox, and the size of the
 jukebox's data if it were to be stored on Venti. The ratio of the size
 of the archival data and the active file system is also given. As can
 be seen, even without using Venti, the storage required to implement
 the daily snapshots in Plan 9 is relatively modest, a result of the
 block level incremental approach to generating a snapshot. When the
 archival data is stored to Venti the cost of retaining the snapshots
 is reduced significantly. In the case of the emelie file system, the
 size on Venti is only slightly larger than the active file system; the
 cost of retaining the daily snapshots is almost zero. Note that the
 amount of storage that Venti uses for the snapshots would be the same
 even if more conventional methods were used to back up the file
 system. The Plan 9 approach to snapshots is not a necessity, since
 Venti will remove duplicate blocks.
 
 Figure 6. Graphs of the various sizes of two Plan 9 file servers.
 
 When stored on Venti, the size of the jukebox data is reduced by three
 factors: elimination of duplicate blocks, elimination of block
 fragmentation, and compression of the block contents. Table 2 presents
 the percent reduction for each of these factors. Note, bootes uses a 6
 Kbyte block size while emelie uses 16 Kbyte, so the effect of removing
 fragmentation is more significant on emelie.
 
 The 10 year history of the two Plan 9 file servers may be of interest
 to other researchers. We have made available per-block information
 including a hash of each block's contents, all the block pointers, and
 most of the directory information. The traces do not include the
 actual contents of files nor the file names. There is sufficient
 information to reconstruct the structure of the file system and to
 track the daily changes to this structure over time. The traces are
 available at http://www.cs.bell-labs.com/~seanq/p9trace.html.
 
 Table 2. The percentage reduction in the size of data stored on Venti.
 
 bootes
 emelie
 Elimination of duplicates
 27.8%
 31.3%
 Elimination of fragments
 10.2%
 25.4%
 Data Compression
 33.8%
 54.1%
 Total Reduction
 59.7%
 76.5%
 
 7. Reliability and Recovery
 
 In concert with the development of the Venti prototype, we have built
 a collection of tools for integrity checking and error recovery.
 Example uses of these tools include: verifying the structure of an
 arena, checking there is an index entry for every block in the data
 log and vice versa, rebuilding the index from the data log, and
 copying an arena to removable media. These tools directly access the
 storage devices containing the data log and index and are executed on
 the server.
 
 The directory structure at the end of each area enhances the
 efficiency of many integrity and recovery operations, since it is
 typically two orders of magnitude smaller than the arena, yet contains
 most of the needed information. The index checking utility, for
 example, is implemented as a disk based sort of all the arena
 directories, followed by a comparison between this sorted list and the
 index. Our prototype currently contains approximately 150 million
 blocks using 250 Gbytes of storage. An index check takes 2.2 hours,
 which is significantly less than the 6 hours it takes to read all the
 log data.
 
 An additional integrity and recovery feature is the association of a
 type identifier with every block. This 8 bit identifier is included
 with all client read and write operations and has the effect of
 partitioning the server into multiple independent domains. The idea is
 that type indicates the interpretation of the data contained in the
 block. A client can use this feature, for example, to indicate that a
 block is the root node for a tree of blocks. Currently, the data
 format associated with a type is left entirely to the client; the
 server does not interpret the type other that to use it in conjunction
 with a fingerprint as the key with which to index a block.
 
 One use of the type identifier is to assist the administrator in
 locating blocks for which a user has accidentally lost the
 fingerprint. Using a tool on the server, the data log can be scanned
 for blocks that match specified criteria, including the block type,
 the write time, and user identifier. The type makes it relatively
 simple to locate forgotten root blocks. Future uses for the type might
 include the ability for the server to determine the location of
 fingerprints within a block, enabling the server to traverse the data
 structures that have been stored.
 
 By storing the data log on a RAID 5 disk array, our server is
 protected against single drive failures. Obviously, there are many
 scenarios where this is not sufficient: multiple drives may fail,
 there may be a fire in the machine room, the RAID firmware may contain
 bugs, or the device may be stolen.
 
 Additional protection could be obtained by using one or more off-site
 mirrors for the server. We have not yet implemented this strategy, but
 the architecture of Venti makes this relatively simple. A background
 process on the server copies new blocks from the data log to the
 mirrors. This copying can be achieved using the Venti protocol; the
 server is simply another client to the mirror.
 
 Even mirroring may not be sufficient. The implementation of Venti may
 contain bugs that can be exploited to compromise the server. An
 automated attack may delete data on many servers simultaneously.
 Storage devices that provide low level enforcement of a write-once
 policy would provide protection for such an attack. Write-once
 read-many optical jukeboxes often provide such protection, but this is
 not yet common for magnetic disk based storage systems. We have thus
 resorted to copying the sealed arenas onto removable media.
 
 8. Related Work
 
 The Stanford Archival Vault [2] is a prototype archival repository
 intended for digital libraries. The archive consists of a write-once
 log of digital objects (files) and several auxiliary indexes for
 locating objects within the log. Objects are identified by the hash of
 their contents using a cyclic redundancy check (CRC). Unlike Venti,
 this system has no way to share data between objects that are
 partially the same, or to build up complex data structures such as a
 file system hierarchy. Rather, the archive consists of a collection of
 separate objects with a limited ability to group objects into sets.
 
 On Venti, blocks are organized into more complex data structures by
 creating hash-trees, an idea originally proposed by Merkle [11] for an
 efficient digital signature scheme.
 
 The approach to block retrieval in the Read-Only Secure File System
 (SFSRO) [3] is comparable to Venti. Blocks are identified by the Sha1
 hash of their contents and this idea is applied recursively to build
 up more complex structures. The focus of this system is security, not
 archival storage. An administrator creates a digitally signed database
 offline. The database contains a public read-only file system that can
 be published on multiple servers and efficiently and securely accessed
 by clients. SFSRO outperforms traditional methods for providing data
 integrity between a client and a file server, demonstrating an
 attractive property of hash-based addressing.
 
 Given their similarities, it would be simple to implement SFSRO on top
 of Venti. The goal of Venti is to provide a flexible location for
 archival storage and SFSRO is a good example of an application that
 could use this capability. In fact, using Venti would provide a
 trivial solution to SFSRO's problem with stale NFS handles since data
 is never deleted from Venti and thus a stale handle will never be
 encountered.
 
 Content-Derived Names [6] are another example of naming objects based
 on a secure hash of its contents. This work addresses the issue of
 naming and managing the various binary software components, in
 particular shared libraries, that make up an application.
 
 The philosophy of the Elephant file system [18] is similar to Venti;
 large, cheap disks make it feasible to retain many versions of data. A
 feature of the Elephant system is the ability to specify a variety of
 data retention policies, which can be applied to individual files or
 directories. These policies attempt to strike a balance between the
 costs and benefits of storing every version of a file. In contrast,
 Venti focuses on the problem of how to store information after
 deciding that it should be retained in perpetuity. A system such as
 the Elephant file system could incorporate Venti as the storage device
 for the permanent "landmark" versions of files, much as the Plan 9
 file system will use Venti to archive snapshots.
 
 Self-Securing Storage [19] retains all versions of file system data in
 order to provide diagnosis and recovery from security breaches. The
 system is implemented as a self-contained network service that exports
 an object-based disk interface, providing protection from compromise
 of the client operating system. Old data is retained for a window of
 time and then deleted to reclaim storage.
 
 Venti provides many of the features of self-securing storage: the
 server is self-contained and accessed through a simple low-level
 protocol, malicious users cannot corrupt or delete existing data on
 the server, and old versions of data are available for inspection. It
 is unlikely that a system would write every file system operation to
 Venti since storage is never reclaimed, but not deleting data removes
 the constraint that an intrusion must be detected within a limited
 window of time. A hybrid approach might retain every version for some
 time and some versions for all time. Venti could provide the long-term
 storage for such a hybrid.
 
 9. Future Work
 
 Venti could be distributed across multiple machines; the approach of
 identifying data by a hash of its contents simplifies such an
 extension. For example, the IO performance could be improved by
 replicating the server and using a simple load balancing algorithm.
 When storing or retrieving a block, clients direct the operation to a
 server based on a few bits of the fingerprint. Such load balancing
 could even be hidden from the client application by interposing a
 proxy server that performs this operation on behalf of the client.
 
 Today, Venti provides little security. After authenticating to the
 server, clients can read any block for which they know the
 fingerprint. A fingerprint does act as a capability since the space of
 fingerprints is large and the Venti protocol does not include a means
 of enumerating the blocks on the server. However, this protection is
 weak as a single root fingerprint enables access to an entire file
 tree and once a fingerprint is known, there is no way to restrict
 access to a particular user. We are exploring ways of providing better
 access control.
 
 To date, the structures we have used for storing data on Venti break
 files into a series of fixed sized blocks. Identical blocks are
 consolidated on Venti, but this consolidation will not occur if the
 data is shifted within the file or an application uses a different
 block size. This limitation can be overcome using an adaptation of
 Manber's algorithm for finding similarities in files [9]. The idea is
 to break files into variable sized blocks based on the identification
 of anchor or break points, increasing the occurrence of duplicate
 blocks [12]. Such a strategy can be implemented in client applications
 with no change to the Venti server.
 
 A more detailed analysis of the decade of daily snapshots of the Plan
 9 file systems might be interesting. The trace data we have made
 publicly available contains approximately the same information used
 for other studies of long term file activity [4].
 
 10. Conclusion
 
 The approach of identifying a block by the Sha1 hash of its contents
 is well suited to archival storage. The write-once model and the
 ability to coalesce duplicate copies of a block makes Venti a useful
 building block for a number of interesting storage applications.
 
 The large capacity of magnetic disks allows archival data to be
 retained and available on-line with performance that is comparable to
 conventional disks. Stored on our prototype server is over a decade of
 daily snapshots of two major departmental file servers. These
 snapshots are stored in a little over 200 Gbytes of disk space. Today,
 100 Gbytes drives cost less than $300 and IDE RAID controllers are
 included on many motherboards. A scaled down version of our server
 could provide archival storage for a home user at an attractive price.
 Tomorrow, when terabyte disks can be had for the same price, it seems
 unlikely that archival data will be deleted to reclaim space. Venti
 provides an attractive approach to storing that data.
 
 11. Acknowledgments
 
 This paper was improved by comments and suggestions from Peter Bosch,
 Eric Grosse, Lorenz Huelsbergen, Rob Pike, Ross Quinlan, and Cliff
 Young and six anonymous reviewers. The paper's shepherd was Ethan L.
 Miller. We thank them all for their help.
 
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