From: Dave Chinner <david@fromorbit.com>
To: Alex Elder <aelder@sgi.com>
Cc: Christoph Hellwig <hch@infradead.org>, xfs@oss.sgi.com
Subject: Re: [PATCH] xfs: remove lazy per-AG initialization
Date: Fri, 4 Jun 2010 11:42:29 +1000 [thread overview]
Message-ID: <20100604014229.GD19651@dastard> (raw)
In-Reply-To: <1275602290.2468.110.camel@doink>
On Thu, Jun 03, 2010 at 04:58:10PM -0500, Alex Elder wrote:
> On Mon, 2010-05-31 at 09:09 +1000, Dave Chinner wrote:
> > On Fri, May 28, 2010 at 01:51:08PM -0400, Christoph Hellwig wrote:
> > > Historically XFS initializes the allocator / inode allocator per-AG
> > > lazily, that is the first time this information is required. For
> > > filesystems that use lazy superblock counters (which is the default now)
> > > we already have to walk all AGs to initialize the superblock counters
> > > on an unclean shutdown.
> >
> > Which is not common, so isn't frequently triggered in the normal
> > mount process. The reason for the lazy initialisation is to speed
> > the mount process up when there are thousands of AGs. That is, we
> > avoid thousands of serialised IOs in the mount path. Have you
> > checked to see what the impact is on the clean mount execution time
> > is on such a filesystem?
>
> It's interesting that the time penalty you're talking about
> doesn't go away, it just becomes less noticeable because it's
> aggregated over subsequent access to the AG's.
Right, the penalty is currently taken at access time, rather than at
mount time. One way to test the impact is to compare the runtime
difference for xfstests with MKFS_OPTIONS="-d agsize=16m" to bump up
the AG count and see how much additional IO and time it takes...
Cheers,
Dave.
--
Dave Chinner
david@fromorbit.com
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prev parent reply other threads:[~2010-06-04 1:40 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-28 17:51 [PATCH] xfs: remove lazy per-AG initialization Christoph Hellwig
2010-05-30 23:09 ` Dave Chinner
2010-06-03 21:58 ` Alex Elder
2010-06-04 1:42 ` Dave Chinner [this message]
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