Re: Fragmentation Issue We Are Having

Dave Chinner
2012-04-12T02:16:32Z

On Wed, Apr 11, 2012 at 06:04:25PM -0700, David Fuller wrote:
> We seen to be having an issue whereby our database server
> gets to 90% or higher fragmentation. When it gets to this point
> we would need to remove form production and defrag using the
> xfs_fsr tool.

Bad assumption.

> The server does get a lot of writes and reads. Is
> there something we can do to reduce the fragmentation or could
> this be a result of hard disk tweaks we use or mount options?
>
> here is some fo the tweaks we do:
>
> /bin/echo "512" > /sys/block/sda/queue/read_ahead_kb
> /bin/echo "10000" > /sys/block/sda/queue/nr_requests
> /bin/echo "512" > /sys/block/sdb/queue/read_ahead_kb
> /bin/echo "10000" > /sys/block/sdb/queue/nr_requests
> /bin/echo "noop" > /sys/block/sda/queue/scheduler
> /bin/echo "noop" > /sys/block/sdb/queue/scheduler

They have no effect on filesystem fragmentation.

> Adn here are the mount options on one of our servers:
>
> xfs rw,noikeep,allocsize=256M,logbufs=8,sunit=128,swidth=2304
>
> the sunit and swidth vary on each server based on disk drives.
>
> We do use LVM on the volume where the mysql data is stored
> as we need this for snapshotting. Here is an example of a current state:
>
> xfs_db -c frag -r /dev/mapper/vgmysql-lvmysql
> actual 42586, ideal 3134, fragmentation factor 92.64%

Read this first:

http://xfs.org/index.php/XFS_FAQ#Q:_The_xfs_db_.22frag.22_command_says_I.27m_over_50.25.__Is_that_bad.3F

Then decide whether 10 extents per file is really a problem or not.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com