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Re: [FAQ] XFS speculative preallocation

To: xfs@xxxxxxxxxxx
Subject: Re: [FAQ] XFS speculative preallocation
From: Arkadiusz MiÅkiewicz <arekm@xxxxxxxx>
Date: Fri, 21 Mar 2014 18:09:03 +0100
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On Friday 21 of March 2014, Brian Foster wrote:
> Hi all,
> Eric had suggested we add an FAQ entry for speculative preallocation
> since it seems to be a common question, so I offered to write something
> up. I started with a single entry but split it into a couple Q's when it
> turned into TL;DR fodder. ;)
> The text is embedded below for review. Thoughts on the questions or
> content is appreciated. Also, once folks are Ok with this... how does
> one gain edit access to the wiki?

More questions or topics that can be converted to questions from me:

1) Before preallocation kernel did things differently. AFAIK it wasn't the 
same as allocsize=64k, was it? Is there a way to get old behaviour or 
something similar to old behaviour?

2) Is there a way to see which file got some preallocation and how big that 
preallocation is? Scenario - something ate free space due to preallocation and 
from admin point of view it would be usefull to know which app did that and 
how many MB was due to preallocation (vs real, written data).

> Linux 3.8 (and later) includes a scanner to perform background trimming
> of files with lingering post-EOF preallocations. The scanner bypasses
> files that have been recently 

What time is "recently" ? Is "modified" equal to "file data modified" or "file 
data or metadata modified" ?

> modified to not interfere with ongoing
> writes.

In case of some app that constantly writes to files (apache web server 
writting to its logs for example) that background trimming will never do 
anything for these files, right?

> A 5 minute scan interval is used by default and can be adjusted
> via the following file (value in seconds):
>       /proc/sys/fs/xfs/speculative_prealloc_lifetime
> 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. Preallocated space can also be
> encoded permanently in situations where file size is extended beyond a
> range of post-EOF blocks (i.e., via truncate). Otherwise, preallocated
> blocks are reclaimed on file close, inode reclaim, unmount or in the
> background once file write activity subsides.

So there is no mechanism that would shirnk preallocations in case when free 
space is (almost or) gone on a fs? Case: apache causes xfs to preallocate 
several GB for its /var/..../{access,error}_log (common problem here) and then 
free space ends on that fs causing problems for every app that writes to /var.


Arkadiusz MiÅkiewicz, arekm / maven.pl

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