From: Eric Sandeen <sandeen@sandeen.net>
To: Dave Chinner <david@fromorbit.com>, Florian Weimer <fw@deneb.enyo.de>
Cc: Brian Foster <bfoster@redhat.com>, xfs@oss.sgi.com
Subject: Re: [FAQ] XFS speculative preallocation
Date: Fri, 21 Mar 2014 18:13:30 -0500 [thread overview]
Message-ID: <532CC79A.8050908@sandeen.net> (raw)
In-Reply-To: <20140321231032.GC1389@dastard>
On 3/21/14, 6:10 PM, Dave Chinner wrote:
> On Fri, Mar 21, 2014 at 09:11:29PM +0100, Florian Weimer wrote:
>> * Brian Foster:
>>
>>> Although speculative preallocation can lead to reports of excess space
>>> usage, the preallocated space is not permanent unless explicitly made so
>>> via fallocate or a similar interface.
>>
>> How does an explicit allocation with posix_fallocate interact with
>> speculative preallocation? Does it disable it?
>
> fallocate is permanent preallocation using unwritten extents.
> Speculative preallocation is an extension of delayed allocation that
> is done when extending the file and the EOF falls into a hole. If
> there is unwritten extents beyond EOF, speulative preallocation is
> not performed.
>
>> I see rather dramatic fragmentation of the systemd journal when it is
>> stored on XFS, and it calls posix_fallocate before writing data to the
>> file.
>
> There's your problem - systemd is preventing delayed allocation, and
> so it fragmenting the file itself with it's write pattern.
> Basically, that's a bug in systemd, and not something the filesystem
> can avoid because userspace is directly controlling block
> allocation.
hohum, I guess we should look into this.
OTOH: nothing wrong with calling posix_fallocate() if you need the space
guarantees it provides for proper operation...
-Eric
> Cheers,
>
> Dave.
>
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next prev parent reply other threads:[~2014-03-21 23:13 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-21 16:29 [FAQ] XFS speculative preallocation Brian Foster
2014-03-21 16:54 ` Shaun Gosse
2014-03-21 17:09 ` Arkadiusz Miśkiewicz
2014-03-21 18:02 ` Brian Foster
2014-03-21 23:16 ` Dave Chinner
2014-03-21 20:11 ` Florian Weimer
2014-03-21 23:10 ` Dave Chinner
2014-03-21 23:13 ` Eric Sandeen [this message]
2014-03-21 23:18 ` Dave Chinner
2014-03-22 13:32 ` Christoph Hellwig
2014-03-21 23:05 ` Dave Chinner
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