[Gllug] A SATA Raid card worth having?

[Russell Howe]

On Thu, Jan 26, 2006 at 03:23:09PM +0000, Ben Fitzgerald wrote:
> on attending a redhat course not long ago an instructor stated that in
> his opinion s/w raid was now as good as h/w raid as:
> 
> a) most cards use crappy processors to perform the parity calculation
> b) modern cpus are very powerful and can easily cope with parity calculation

The CPU speeds these days probably mean the RAID calculations aren't
going to kill them. I guess the main problem is the shift of I/O
bottlenecks..

With a hardware RAID card in a PCI slot, you do your I/O to the PCI
card, and then it farms out the relevent I/O to whatever disks it has
attached.

With software RAID, the OS needs to do all the I/O itself, and in all
the boxes I've used, that would mean sharing a single PCI bus to access
either the same card multiple times, or multiple cards. I can't remember
what the bandwidth of PCI is, but it's not hugely fast.. If you're doing
RAID1, you effectively cut the PCI bandwidth available for disk
operations in half (since you'll be writing everything twice).

It does mean you have to think about this when speccing a system, and
probably means software RAID on a box which wasn't built with it in mind
isn't going to offer stunning performance.

Also, what if your OS and bootloader are installed on a RAID1 set and one
of the drives dies? Can the box boot up successfully with a dead boot
disk, bypassing it and booting off of the remaining good one? I suspect
most if not all PC BIOSes would have problems here, depending on just
how the drive had died. Maybe the software RAID cards have more
intelligent BIOSes than your typical SCSI/IDE/SATA-supporting
motherboard?

I've never used software RAID - I'm trying to get a box set up with
hotswap SCA drive bays, but so far FreeBSD and NetBSD haven't handled
hotswapping the drives at all well. I was hoping to go BSD on this box,
but it looks like I might have to try Linux on it instead. It's made
more interesting by the fact that it's a DEC Alpha :)

> Others may disagree, I am just the messenger (tm). Don't flame me but I
> would be interested to read comments.

-- 
Russell Howe       | Why be just another cog in the machine,
rhowe at siksai.co.uk | when you can be the spanner in the works?
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