[mythtv-users] Backend with storage group on raid array

Andy Burns mythtv.lists at burns.me.uk
Sat Oct 15 14:14:56 UTC 2011

I have a dedicated backend, its single storage group is on an mdadm
RAID5 array of 8x SATA II 750GB disks, not all the space belongs to
myth, just a 1TB LVM volume, there are other volumes, The system disks
and mySQL are on another RAID1 array of 2x SATA II 250GB disks.

This worked OK with SD, two tuners with four way multirec could be in
operation with playback and chaseplay still being smooth, I now have
two HD tuners and this has rocked the balance somewhat.

If I make a recording and then watch it after the recording is
finished, it works fine, 2MB/s of disk reads while watching.

If I watch live TV with the chaseplay keeping closely behind the
recording point, it also works fine, 2MB/s of disk writes, the
playback comes "for free" from the linux caching layer

However if I'm using chaseplay with a several minutes delay, or
watching one recording while making another, the playback stutters,
presumably due to competing access to the disks (even though this is
only about 2MB/s of reads plus 2MB/s of writes, yet the disks are
capable of about 330MB/s aggregate reads e.g. during RAID rebuilds

There are a few things I could try ...

Create a small live tv storage group LVM volume on the RAID1 array
(this will then be sharing spindles with mySQL) and move recordings to
the 1TB group if required for long term storage.

Or, recreate a smaller array with 7x disks, use 1 TB of that for long
term storage, and my other LVM solumes, and then have a single 750GB
disk dedicated to live TV, which wouldn't have the RAID overhead

Or, tinker with RAID5 block sizes to see if that can benefit myth, but
it might be to the detriment of the other LVM volumes.

Any thoughts?

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