Re: [PATCH] xfs: remove lazy per-AG initialization

[Alex Elder <aelder@sgi.com>]

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.

I like the cleanup too, but I agree it would be useful to
quantify what this particular impact is.

					-Alex

> FWIW, in the case of an unclean shutdown, we are already on the slow path
> due to log recovery so adding IO to read all the headers it not such
> a big deal as they have probably been read in during replay, anyway.
> 
> > This patch generalizes that code so that we
> > always initialize the per-AG data on mount, and also during growfs so
> > that we can remove all the special case code in the fastpath which
> > couldn't assume that the per-AG data is already initialized.
> 
> I like the cleanup, but I'm not sure that potentially adding tens of
> seconds to the time to mount a really large filesystem is a good
> tradeoff...
> 
> Cheers,
> 
> Dave.



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