[mythtv-users] Recording directly to RAID

[Tapani Tarvainen]

On Fri, Jul 27, 2012 at 06:58:40PM -0400, Joseph Fry (joe at thefrys.com) wrote:

> First of all... with 4 drives, a RAID 10 or 01 would  be best.

Yes. (With Linux mdadm, RAID10, period. With hardware RAID, it depends.)
In particular, RAID5 and RAID6 are poor choices for any situation where
write speed is critical (and even RAID10 needs non-default parameters
to do well).

> But do yourself a favor and DONT USE RAID.
>
> I ran a variety of RAID configurations over the years and have finally seen
> the error of my ways.

> Mythtv works best with multiple drives in it's storage groups and does a
> wonderful job spreading the load out.

True: MythTV does work quite well with independent disks, and for
MythTV-only use RAID may not pay off. But it's not quite that
clear-cut as you suggest.

> With my arrays, I would hear the drives thrashing as I recorded 2-3
> channels, watched one recording, and had a commflag running, the
> heads were all over the disks; because the array used all of the
> disks for each of 5 processes.

That can happen - getting a RAID array to be efficient with
multiple processes takes some tuning, you need more RAM
for buffering and good SATA card(s).

But my 5-disk RAID10 server can handle everything I can throw
at it over gigabit network, including several mythtv recordings
at the same time with lots of other stuff going on.
There's no way I could've gotten similar performance without RAID.

If it was dedicated to MythTV use, it might not be worthwhile,
especially if you're not managing RAID arrays otherwise,
as it does add complexity to the system and getting it to
run well takes time and effort.

> Now, with 4 independent disks, typically each disk is handling one
> or 2 of those processes... which rarely taxes the drive.

Yes. For MythTV that should work well.
It is also very simple setup.

> Additionally, software raid has significant overhead... so a few percent of
> my processor was dedicated to just handling the IO.

That is hardly significant unless you're using Atom or similar
ultra-low-power CPU.

> Finally, RAID does nothing for you if your data is corrupted.

> If you have 4 drives and are willing to lose 50% for redundancy... a real
> backup is a far better use of that extra space than RAID will ever be.

Yes. RAID is no substitute for backups. But the opposite is also true.

What RAID gives you that no backup can is uptime: recording won't
stop because one disk dies. And that is really the only good reason
for considering RAID in a dedicated mythtv box.

RAID can also give serious performance boost, but as noted that takes some
tuning and in a mythtv-only setup set of single disks will do quite
well without any effort.

-- 
Tapani Tarvainen