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Re: XFS fragmentation on file append

To: Keyur Govande <keyurgovande@xxxxxxxxx>
Subject: Re: XFS fragmentation on file append
From: Stefan Ring <stefanrin@xxxxxxxxx>
Date: Thu, 24 Apr 2014 08:54:54 +0200
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, Linux fs XFS <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
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I've become interested in this topic, as I'm also running MySQL with
O_DIRECT and innodb_file_per_table. Out of curiosity, I immediately
ran xfs_bmap on a moderately sized table space (34GB). It listed
around 30000 fragments, on average one for every MB.

I want to report what happened then: A flurry of activity started on
both disks (root/swap lives on one of them, the data volume containing
the MySQL files on another) and lasted for about two minutes.
Afterwards, all memory previously allocated to the file cache has
become free, and also everything XFS seems to keep buffered internally
(I think it's called SReclaimable) was released. Swap usage increased
only slightly. dmesg was silent during that time.

This is a 2.6.32-358.2.1.el6.x86_64 kernel with xfsprogs 3.1.1 (CentOS
6.4). The machine has 64GB of RAM (2 NUMA nodes) and 24 (virtual)
cores. Is this known behavior of xfs_bmap?

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