[mythtvnz] Myth & Raid. SW or HW..

[Toby Mills]

>Many seem to take the view that there is little point to RAID since primary
>storage is for TV recordings, which I agree are low value. However, in my
case
>the bulk of my data is not taken up with TV recordings but movies and
music,
>which I consider very high value since I don't know how many hours of
labour I
>put in ripping my rather huge DVD and CD collection.

This is a good point about videos and music, its wise to remember that RAID
is not a backup strategy.
RAID is a method of avoiding downtime and increasing performance, however
in a myth setup, there is a relatively low cost of downtime so RAID has a
lower value.

I take the same view on Videos and Music, however I don't safeguard this
with RAID.
I simply use the same 1Tb USB drive that I use for configuration backups as
my primary storage of Videos and music, but then use RSYNC to take a weekly
copy of this onto another 1Tb USB drive.

At the end of the day, you don't need the performance of RAID for your
Video and Music drive. A USB drive can easily handle playback of multiple
DVD's. The plus side is that it is also portable should you need it to take
it somewhere. Your backups also don't need to be online, once a week is
usually fine for Videos and Music as they do not change frequently.

I'm a big fan of RAID in large storage situations concerning production
databases and mission critical systems, but it has very little value in a
myth setup and simply serves to complicate matters and reduce performance.

The key issue with RAID for mythtv is fragmentation, fragmentation can
offset the increased perofmance you get from having raid.
The file I/O profile of a mythtv setup is quite different from what RAID
was designed and optimized for.
This is why the myth dev team created storage groups.
If you have 4 recordings being recorded to a raid array at the same time,
then the recordings will not be contiguous, this results in a very high
level of fragmentation. If you divide your raid array up into storage
groups to get around this, then if your array goes down you have a 100%
outage or degradation while it is repaired.
This makes no sense when mythtv has a built in method of maximising
performance and redundancy.

If you used the same disks in your raid array to create 1 storage group
from each disk then your performance will be better than any RAID
configuration and your risk of data loss will be lower than every single
raid configuration except RAID 10. You will also have more storage capacity
than any RAID configuration.

You can also take storage groups offline and replace disks while people are
still watching TV (so long as it is not a program that is on the spindle
you are replacing).

Toby
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