Fragmentation Issue We Are Having

[Eric Sandeen]

On 4/11/12 9:55 PM, David Fuller wrote:
> Dave,
> 
> Thanks so much for that informative read.  This helps me fight my case that systematic
> defrags are not needed and are bad for the system in general.
> 
> After reading this I did do some checks against some of our larger tables and found that
> on average we are storing about 2.5GB per extent. For me that seems pretty reasonable
> to me and does not require defrag'ing at this time.

I've also added a visual aid to that faq entry to show how quickly the frag factor
approaches 100% :)

-Eric

> --David Fuller
> 
> 
> <http://www.topshelfads.com/email/sig/epochbestbilling.jpg>
> 
> 
> On Wed, Apr 11, 2012 at 7:16 PM, Dave Chinner <david at fromorbit.com <mailto:david at fromorbit.com>> wrote:
> 
>     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 at fromorbit.com <mailto:david at fromorbit.com>
> 
> 
> 
> 
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