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From Stefan Sperling <s...@elego.de>
Subject Re: Support for filesystem snapshots (?)
Date Mon, 02 Aug 2010 16:17:39 GMT
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.

Stefan

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