Access to UFS stuck during snapshot.

[Kris Kennaway]

On Wed, May 24, 2006 at 07:18:00AM -0500, Eric Anderson wrote:
> Kris Kennaway wrote:
> >On Wed, May 24, 2006 at 02:17:51PM +0800, Kent Ho wrote:
> >>Hi,
> >>
> >>I having problems accessing file system during snapshot runs.  snapshot 
> >>takes approximately 10 to 20 mins to run.
> >>
> >>During that time access to files in the file system are blocked until 
> >>the snapshot has completed.  Lot of processes was stuck and piles up on 
> >>the fs.
> >>
> >>This fs stores mailboxes and it's accessed 24/7.  This is similar to 
> >>what described in this thread.
> >>
> >>http://www.freebsd.org/cgi/getmsg.cgi?fetch=14968+0+/usr/local/www/db/text/2006/freebsd-fs/20060312.freebsd-fs
> >>
> >>Has this been improved or fixed in recent releases, I see a bunch of 
> >>fixes in march?  any possible work around?  any alternatives or 
> >>recommendations?  Looking for high availability fs with snapshots.
> >
> >It's by design.
> >
> >Kris
> 
> I found that making the .snap directories permissions 0700, and then 
> making a subdirectory under the .snap that stores the actual snapshots 
> helped, as long as I did not make any stat() calls to the snapshot file 
> that was in snaplk state.
> 
> Kris - you've had your head deep in the snapshot spaghetti recently - is 
> this blocking just a matter of the locking on the snapshot file and a 
> stat() call blocking on it, which causes an upward locking to the root 
> dir of the filesystem?  Could this be avoided by 'hiding' the snapshot 
> file while a snapshot is in progress?  I have been tinkering with hiding 
> the snapshot directory during snapshot creation, but I don't have any 
> results of anything yet, still too early.  I'm a wannabe hacker, so my 
> progress is slow.

No, I/O is suspended while the snapshot is being created.  As I said,
it's a design limitation.

Kris
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