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From "Vallon, Justin" <Justin.Val...@deshaw.com>
Subject RE: Support for filesystem snapshots (?)
Date Mon, 02 Aug 2010 16:42:31 GMT
I did see that discussion, but it seems to contradict with the claim that the database operations
are transactional.

So, my follow up is:  If I unplug the network cable between svn-commiter and filesystem, will
the repository be corrupt?

If yes, then the underlying database is not atomic and therefore not transactional.

If no, then the server can take a snapshot at any point in time, and the snapshot is guaranteed
to be consistent.

-- 
-Justin


-----Original Message-----
From: Les Mikesell [mailto:lesmikesell@gmail.com] 
Sent: Monday, August 02, 2010 12:30 PM
To: users@subversion.apache.org
Subject: Re: Support for filesystem snapshots (?)

On 8/2/2010 11:17 AM, Stefan Sperling wrote:
> On Mon, Aug 02, 2010 at 11:38:30AM -0400, Vallon, Justin wrote:
>> In the svn book, http://svnbook.red-bean.com/en/1.1/ch05s03.html#svn-ch-5-sect-3.6
on "Repository Backup" describes some backup methods - hot copy, etc.  Given that enterprise-level
filesystems generally support filesystem snapshots, what is SVN's position on whether such
snapshots are sufficient for backups?
>>
>> Suppose our criteria is that we unplug the network cable between the writer (svn
commit) and the filesystem.  If unplugging the network cable could yield a repository that
is corrupt, then doesn't that mean that there are failure modes where the repository is corrupted?
 If a filesystem insures that the snapshots behave in this way (as if you disconnect the network
cable, make an instant copy, then reconnect), it would seem to imply that these volume-level
snapshots would result in a consistent view.  If the snapshots are not consistent, then atomicity
is not being insured (somewhere between application, client o/s, server o/s, disk controller,
media).
>>
>> My follow up is whether the svn book could include a word about whether it blesses
(or not) this form of backup?
>
> By chance, this very topic was discussed earlier today.
> See http://svn.haxx.se/users/archive-2010-08/0019.shtml
>
> Quote:
> "Use 'svnadmin hotcopy' to create a copy of a repository that is consistent
> from Subversion's point of view. There is no other repository copying
> tool that can guarantee a consistent repository snapshot."
>
> This holds true for filesystem-level snapshots.
> The atomicity of Subversion's commits only exists from the application's
> point of view.

Are there any svnadmin commands that could quiesce/lock the repository 
in a consistent state for the duration of a filesystem snapshot - or if 
not, could that be added as an enhancement?

-- 
   Les Mikesell
    lesmikesell@gmail.com

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