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Re: XFS Preallocate using ALLOCSP

To: Eric Sandeen <sandeen@xxxxxxxxxxx>
Subject: Re: XFS Preallocate using ALLOCSP
From: Felix Blyakher <felixb@xxxxxxx>
Date: Tue, 16 Jun 2009 17:32:08 -0500
Cc: Smit Shah <getsmit@xxxxxxxxx>, linux-xfs@xxxxxxxxxxx
In-reply-to: <4A38171A.4080500@xxxxxxxxxxx>
References: <24042506.post@xxxxxxxxxxxxxxx> <4A3712BF.7030101@xxxxxxxxxxx> <8770d98c0906152344p185533a9rc144a5667d13d2de@xxxxxxxxxxxxxx> <4A37B744.9030301@xxxxxxxxxxx> <0B774481-16A5-42FC-89C3-91096E59E861@xxxxxxx> <8770d98c0906161028j1cc5cbadl49d30092fddf3dbe@xxxxxxxxxxxxxx> <B7C1A222-58C5-4858-80B4-F871BF088DB1@xxxxxxx> <8770d98c0906161442t634467bxe8b0f5c32b49502e@xxxxxxxxxxxxxx> <4A38171A.4080500@xxxxxxxxxxx>

On Jun 16, 2009, at 5:05 PM, Eric Sandeen wrote:

Smit Shah wrote:

Since fallocate uses the RESVSP cmd for xfs. And as given given for
RESVSP in man page for xfsctl
If the XFS filesystem is configured to flag unwritten file extents,
performance will be negatively affected when writing to preallocated
space, since extra filesystem transactions are required to convert
extent  flags  on  the  range  of  the  file  written.

And ext4 must do basically the same thing, as would any fs that flags
unwritten extents.

ext4 may convert more at a time, though, rather than leaving
"fragmented" written/unwritten/written/unwritten regions.

Converting more at a time would require writing extra zeros
to extra space converted, but not covered with write. Again,
it's a trade off, and there is no clear winner. Well, at least
not in my mind, until proven that some operations are more
efficient than others.


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