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Date: Sat, 6 Aug 2011 02:24:15 -0400
From: Gary Palmer <gpalmer@freebsd.org>
To: Doug Barton <dougb@FreeBSD.org>
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Cc: freebsd-stable@freebsd.org
Subject: Re: ZFS directory with a large number of files
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On Fri, Aug 05, 2011 at 08:56:36PM -0700, Doug Barton wrote:
> On 08/05/2011 20:38, Daniel O'Connor wrote:
> 
> > Ahh, but OP had moved these files away and performance was still poor.. _that_ is the bug.
> 
> I'm no file system expert, but it seems to me the key questions are; how
> long does it take the system to recover from this condition, and if it's
> more than N $periods is that a problem? We can't stop users from doing
> wacky stuff, but the system should be robust in the face of this.

Its been quite a while since I worked on the filesystem stuff in any
detail but I believe, at least for UFS, it doesn't GC the directory,
just truncate it if enough of the entries at the end are deleted
to free up at least one fragment or block.  If you create N files and
then a directory and move the N files into the directory, the directory
entry will still be N+1 records into the directory and the only way to
"recover" is to recreate the directory that formerly contained the N
files.  It is theoretically possible to compat the directory but since 
the code to do that wasn't written when I last worked with UFS I suspect
its non trivial.

I don't know what ZFS does in this situation

Gary