[mythtv-users] Linux software raid question

Yan-Fa Li yanfali at gmail.com
Thu Jun 5 17:55:51 UTC 2008


On Thu, Jun 5, 2008 at 10:34 AM, David Brodbeck <gull at gull.us> wrote:
> 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.

This is definitely an issue with older PCI based systems.  PCI-X
scales to 133MHz and has lots of bandwidth.  For home use, PCIe is
cheap and widely available.  PCIe 4X slots should be more than fast
enough for disk I/O as this is effectively the same bandwidth as a
66MHz/64bit PCI-X slot.  Even PCIe 1X is better than old PCI because
there's no bus contention as it's a switched based architecture.  This
is a great time to be building software based RAID systems on the
cheap.  A lot of pieces are finally moving into place like hot
plugging, Port Multipliers and a decent and inexpensive disk standard
SATA.

Yan


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