OT: Single-assignment, was Re: Raskin book on UI and radical data thoughts

Scott Lystig Fritchie fritchie@REDACTED
Thu Apr 3 22:22:21 CEST 2003

I'm sorry, I just couldn't keep silent, even though this is off-topic.
Maybe it will be interesting to somebody in the same manner that Marc
Ernst Eddy van Woerkom's message about the "Judy tree" interested me.

>>>>> "cp" == Chris Pressey <cpressey@REDACTED> writes:

>> An interesting approach would be to make the disk
>> single-assignment.  *Never* allow data to be modified, only allow
>> new versions of the data to be created and retain all versions for
>> their lifetime.

cp> I think we already have these: journalling file systems?

There is at least one write-once file system that I'm aware of: Venti.
There's a USENIX paper at
http://www.usenix.org/events/fast02/quinlan.htm that describes it.  At
USENIX 2002 I heard Rob Pike talk a little bit about it.  IIRC, during
early development, they'd make "backup" snapshots of the entire file
system by writing the SHA1 hash (20 bytes) of the root directory on a
whiteboard ... that was all the information they needed to find
everything else in the file system.


--- snip --- snip --- snip --- snip --- snip --- snip --- snip --- 

FAST 2002 Abstract 

Venti: a new approach to archival storage 

Sean Quinlan and Sean Dorward, Bell Labs, Lucent Technologies 


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.

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