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List:       freebsd-hackers
Subject:    Re: UFS Snapshots and iowait
From:       krad <kraduk () gmail ! com>
Date:       2010-11-23 22:02:30
Message-ID: AANLkTin7mSuQXUc==7CD=BvAPHK4J4d5gmPbmtPe-Bq0 () mail ! gmail ! com
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On 23 November 2010 19:49, Kurt Lidl <lidl@pix.net> wrote:

> On Tue, Nov 23, 2010 at 10:38:31AM -0800, Chris St Denis wrote:
> > Is this just due to the very high io bandwidth usage associated with
> > making a snapshot, or does the creation of this snapshot completely
> > block IO writes for around 5 minutes?
>
> It blocks updates to the filesystem while during part of the
> snapshot process.
>
> See the comments in /usr/src/sys/ufs/ffs/ffs_snapshot.c
>
> I found using UFS snapshots on a production fileserver untenable
> during normal working hours.  I have a backup fileserver that I
> rsync the files to, and then use the UFS snapshots there.
>
> > Any suggested workarounds? I already bumped up the number of Apache
> > slots to 166% but it looks like I would have to increase the number much
> > more to use that as a primary solution.
>
> Use ZFS.  The way snapshots work there, they are nearly instantanous
> to create, and you are not limited to 20 snapshots per filesystem.
>
> -Kurt
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>

I can testify zfs snapshots are very usable, as we use it to backup our
mysql and oracle databases. Issue a write lock, flush, snap, remove lock,
backup snapshot All takes a few seconds and is fairly seamless
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