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Re: understanding speculative preallocation

To: Jason Rosenberg <jbr@xxxxxxxxxxxx>
Subject: Re: understanding speculative preallocation
From: Ben Myers <bpm@xxxxxxx>
Date: Fri, 26 Jul 2013 15:50:18 -0500
Cc: stan@xxxxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
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User-agent: Mutt/1.5.20 (2009-06-14)
Hi Jason,

On Fri, Jul 26, 2013 at 04:38:21PM -0400, Jason Rosenberg wrote:
> Thanks for the info (most of it was, in fact, news to me).  I'm an
> application developer trying to debug a disk space problem, that's all.  So
> far, I've tracked it down to being an XFS issue.

The speculative block reservations have been an issue for awhile.  You are not
the first person to take issue with it.

> Regardless, is there a version history for XFS vis-a-via mainline Linux?

You can find a full version history for XFS back to 2.6.12 or so here:
http://oss.sgi.com/cgi-bin/gitweb.cgi?p=xfs/xfs.git;a=summary

If you're interested in going older than that look here:
http://oss.sgi.com/cgi-bin/gitweb.cgi?p=archive/xfs-import.git;a=summary

The function you'll most likely want to track is xfs_iomap_write_delay, which
calls xfs_iomap_eof_want_preallocate, both of which are in fs/xfs/xfs_iomap.c.

Recently Brian Foster added a scanner to remove the speculative block
reservations on a timer which may give you some relief.  See
xfs_queue_eofblocks in fs/xfs/xfs_icache.c

Regards,
        Ben

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