Revfile vs Atomicity & Dumbfs

[John A Meinel]

Matt Mackall wrote:
> On Mon, May 09, 2005 at 05:01:53PM -0500, John A Meinel wrote:
>
>>I've been reading through the bzr docs, (I've gotten through the mailing
>>lists, and I'm working on the inside documentation).
>>
>>Generally, I like what I'm seeing, but I'm concerned about 2 things,
>>both stemming from the same source.
>>
>>Right now, I think you are just keeping a complete copy of each revision
>>of a file, which you obviously don't want to do over time. The current
>>suggestion is to use the "revfile" method, which has an append-only
>>index and an append-only text store.
>>
>>The thing is, append-only isn't very transaction safe, it's certainly
>>better than write anywhere, but new-file only works better with backups,
>>and atomicity. And unless I'm mistaken, it is easier to add a new file
>>to a remote connection, than it is to append to an existing one (at
>>least with sftp/ftp, webdav may be different).
>
>
> Mercurial, which is where the revfile idea comes from, has a very
> simple transaction and rollback scheme. Not sure how it interacts with
> network protocols though. Mercurial treats network sources as
> read-only.
>

Well, I got this from here:
http://www.selenic.com/mercurial/notes.txt

> Rollback:
>
> Rollback is not yet implemented, but will be easy to add. When
> performing a commit or a merge, we order things so that the changeset
> entry gets added last. We keep a transaction log of the name of each
> file and its length prior to the transaction. On abort, we simply
> truncate each file to its prior length. This is one of the nice
> properties of the append-only structure of the revlogs.
>

Which basically means they use WAL.
I don't know that all filesystems support truncate. But it's probably a
decent enough rollback mechanism.
I'm still not sure how dumbfs support would work, it seems like you
would have to transfer the entire revfile, rather than being able to append.

John
=:->

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