[mythtv-users] Newbie

Simon Hobson linux at thehobsons.co.uk
Fri Dec 2 22:54:49 UTC 2011

Eric Sharkey wrote:

>  >> Don't raid or stripe your recording disks together, you'll get better
>>>  performance with individual disks.
>>  I'm in the process of building a MythTV combined FE/BE and I had thought
>>  to put in 2 x 2TB SATA disks for the recordings and video/music
>>  library.  I was going to use RAID1 (mirror) so that a single disk
>>  failure didn't mean I loose all my recordings and ripped videos/music.
>>  But you seem to be advising against this approach.  How do you protect
>>  against data lose if not via RAID?
>He said performance, not reliability.
>Your choices are either backup or prayer.  For mythTV recordings, I
>just keep my fingers crossed.  I lost a recording drive once.  The
>world didn't end.

Some people have suggested striping their disks into one virtual 
volume. This may be better for some tasks, but because of the way 
Myth works, it will be significantly inferior for Myth. As i said, 
the primary constraint for most people seems to be when handling 
multiple streams - and disk seeks are the killer there. If you stripe 
your disks, then you are guaranteed that all the active streams will 
be operating on both disks - and the slowest seek time will set the 
overall performance, effectively the performance of the array will 
never be higher than any of the disks.

If you use the disks separately (Myth handles an arbitrary set of 
recording directories), then most of the time you will have the 
streams spread across the drives. By default, Myth will try and 
balance recording jobs across all the recording directories. Thus the 
throughput available to you will be much higher.

Eg, if one drive could handle 3 streams, then 2 drives could handle 
6, 3 drives could handle 9. Obviously it's not as simple as that as 
you could end up with (say) 6 streams split 2 & 4 across 2 drives - 
but you'd be unlucky to have (say) 4 streams on one drive and none on 
the other.

Also, if you have multiple drives, then you don't lose all your 
recordings if one dies.

As to mirroring two drives. I suspect the throughput would be fairly 
close to the throughput available on the slowest drive.
Simon Hobson

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