[mythtv-users] RAID 1 for recordings and livetv

[Joseph Fry]

> I am configuring my new backend, I want to know if there is any real
>> performance if I create a RAID 1 disk to store the recordings and
>> livetv files.
>>
>> I don't worryed about data lose, by the way, there are just TV programs.
>>
>> I will appreciate your experience on RAID systems.
>>
>> Best regards.
>>
>> --
>> Josu Lazkano
>>
>
> The general consensuses from people on the list (from what I have seen
> previously) is no, you won't see any noticeable gains and would be better
> using all of the space as separate disks for recordings.
>
> The mysql database however, may benefit from being stored on a separate
> disc at least.  My stuff is all of the same disc, but my usage of myth is
> small, compared to some people on the list.
>
> Did you mean RAID1 by the way, which is just a mirror?  Or did you mean
> RAID0, which is striped.
>
>
For the type of loads that Mythtv generates, no raid solution will
outperform an equal number of individual disks with their load properly
balanced.  For example, if recording two streams to two disks:

- If raided (raid 0 or 1), the heads of both drives will be seeking back
and forth between the two locations on disk to write each stream.
- If individual disks, the recordings are written sequentially on each
disk, with minimal head movement.

If you have more than 2 sequential IO's (say your watching a show,
recording 2, and commflagging), your still keeping seeks to minimum because
if raided, it needs to seek between 4 positions on both disks, rather than
seeking between 2 locations on each disk.  Seeks are the slowest part of
any magnetic storage solution, which is why flash media is so much faster;
RAID can make it worse because read/writes cannot occur until the last
drive arrives at the proper track/cylinder.  (slight oversimplification)

Where RAID can increase performance is when doing single sequential tasks,
or many small reads that can be fulfilled by a single drive of the array
(databases and webhosting on a properly tuned array).  Hardware raid uses
large caches to offset the performance losses associated with raid... so
often a hardware raid solution will be faster in many circumstances...
though often not with multiple sustained sequential operations like we see
with mythtv which would rapidly fill the cache.

In summary.. use raid if you need redundancy or if your system is serving
another role that may benefit from raid (large databases, webhosting, etc)
otherwise, just use the extra drive capacity you would lose in an array to
store online backups of your most important data so you can quickly recover
from a drive failure.

Just my recommendation... I'm sure others would argue otherwise.
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