[mythtv-users] Linux software raid question

David Brodbeck gull at gull.us
Thu Jun 5 17:34:31 UTC 2008

Dan Ritter wrote:
> On Wed, Jun 04, 2008 at 02:49:11PM -0400, Carl L. Gilbert wrote:
>> Hardware RAID does not have to be expensive.
> Of course it does. Let's say that software RAID takes up 1% of
> your CPU. 1% of a $200 CPU is $2. There are no reliable hardware
> RAID systems that cost $2.

This is a good point for small systems.  One caveat is that, while the
CPU load is low, software RAID can run into bus contention problems for
really big arrays.  Imagine you have a 10-disk array.  When you write to
it, individual commands have to go across your PCI bus to 10 different
disks.  With a hardware controller, the write goes out on the bus once,
and the rest happens internally.  There is a point where this difference
becomes important, but I don't know how many disks you have to have
before it does.  It depends partially on what else you've got going on
on the bus.  (For example, multiple frame-grabber cards can saturate a
PCI bus very quickly.  So can gigabit ethernet.)  I know that *IDE* bus
contention was a performance problem for RAID arrays that contained
multiple IDE drives on the same cable.

I suspect this is a non-issue for home systems because they don't have
arrays that are large and/or don't care much about performance.

> If the card blows, you had better have a replacement on hand of
> exactly the same type. Not so for software RAID. And if the card
> blows after warranty or after the manufacturer has EOL'd it, you
> might be looking at a very expensive replacement.

3ware, from what I've seen, seems to be pretty good about
back-compatibility and cross-compatibility.  I agree this can be a
problem if you get the card from a manufacturer that goes belly-up,
turning it into abandonware.  But really, in that case the proper
response is probably to restore from your backup, not to spend $$$$ on
trying to find the same card somewhere else.  (See my other post about
RAID not being a substitute for backups.)

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