[mythtv-users] RAID 1 for recordings and livetv

[Simon Hobson]

Joseph Fry wrote:

>- If individual disks, the recordings are written sequentially on 
>each disk, with minimal head movement.
...
>Just my recommendation... I'm sure others would argue otherwise.

No, seems a reasonable "layman's guide". Some slight 
corrections/observations though :

When writing a recording, the file is synced about once/second. That 
involves a write of the data and then a write of the metadata. So 
each second there will be as a minimum "seek-write-seek-write" per 
stream. Some of the metadata writes will involve multiple seeks - 
perhaps reading some data on free space, updating that (and it's 
metadata), and then writing the data.
So it's actually worse than the "one seek per thread per second" suggested.

RAID1 can turn in excellent read performances - potentially much 
higher than a single drive. In the absence of writes, an intelligent 
controller can distribute read requests across the two drives 
according to their head locations - so seek distances can be 
significantly reduced on both drives and it's possible to think of a 
contrived (and artificial) situation where a 2 disk stripe could give 
more than twice the read performance of each disk individually.
Of course, throw in writes and both disks must seek to the same place 
- but then can seek off in different directions to satisfy reads from 
either end of the mirrored volume.
But I suspect it's going to be rare where the advantages of a striped 
volume outweigh the downsides for Myth use.
-- 
Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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