Introduction & background

Archival storage is a second class functionality for current computer environments.

The storage capacity exceeds the ability of many users to generate data, making it practical to archive data in perpetuity.

Write-once policy

Introduction & background

A prevalent form of archival storage is magnetic tape.

Restoring data from a tape can be tedious and error prone.

A trade off exist between performance of backup and restore operations.

Introduction & background

Snapshots avoid the trade off between full and incremental backups.

The Venti Archival Server

Venti is a block-level network storage system.

It identifies data blocks by a hash of their contents.

Write-once, data replication is idempotent.

The Venti Archival Server

Using magnetic disks for Venti has the benefit of reducing the disparity in performance between conventional and archival storage.

Applications

Applications use the block level service provided by Venti to store more complex data structures.

Vac is an application for storing a collection of files and directories as a single object. 

VAC

Applications

In this alternative, the disk blocks that make up the file system are directly copied without interpretation. This enables simplicity and potentially much higher throughput.

Physical backup

Applications

The new version of Plan 9 uses Venti instead of an optical jukebox. This equalizes access to active and archival view of the file system.  It also allows the cache to be quite small.

Plan 9 file system

Applications

Implementation

The implementation uses an append-only log of data blocks and an index that maps fingerprints to locations in this log. One main goal of the prototype is robustness.

To ease maintenance, the log is divided into self-contained structures called arenas. Each arena contains a large number of data blocks and is sized to facilitate operations such as copying to removable media. 

Implementation

Implementation

Client

Client

Client

Network

Block Cache

Index Cache

Index

Implementation

data log

arena

Data blocks

header

Directory

Trailer

Implementation

Performance

The uncached sequential read performance is

particularly bad. The problem is that these sequential reads require a random read of the index.

Performance

Performance

Performance

Reliability & Recovery

Integrity checking and error recovery is of fundamental importance. There are several tools implemented along with Venti to achieve this: verifying the structure of the arena, checking a one on one relation between data blocks and entries in the data log and copying an arena to removable media. 

Reliability & Recovery

There is also a type identifier associated with each block, this integer is included in every write or read operation and has the effect of partitioning the server into multiple independent domains.  

Related & Future work

There are several systems similar to Venti. For example the Stanford Archival Vault, that unlike Venti, it has no way to share data between objects that are partially the same. Another system is the Read-Only Secure File System though, the focus of this system is security and not archival storage. Finally the Elephant file system, could incorporate Venti as the storage device for the permanent versions of  files.

Related & Future work

• Venti could be distributed across multiple machines


• Venti provides little security


• Similarities on files (what if a data block shifts)

Conclution & Critic

•  The use of disk technologies overly complicates random reads and writes, maybe solid state technologies should be explored.
•  The performance penalty of archiving new data blocks is too large for a system whose logic is perpetual archival of information.
•  The authors say that performance is not as good as they expected, but that the results look promising, whereas,  they do not abound in the reasons why they say so.
• It is not easy to implement on any given application, the application must be suitable for Venti's properties and behavior.
• Full index checkup implementation is frankly naïve.