Re: [tlug] SATA software RAID or SAS hardware RAID?

[Jonathan Byrne <jq@example.com>]

On Saturday 09 December 2006 23:49, Curt Sampson wrote:

> Sad to say, copper SCSI doesn't really offer much advantage these
> days.

Well, maybe one: SCSI disks are still more reliable than [PS]ATA disks, 
although those have gotten so reliable that this my no longer be an 
issue for most people.

Another is that you can put several SCSI devices on a cable and still 
get good performance b/c the SCSI host adapter offloads the work from 
the CPU. Serious server applications still tend toward SCSI (at my last 
gig, we had 500+ quad-Xeon servers, all running SCSI hardware 
accelerated RAID).

As you mentioned in an earlier post on this topic, the issue of 
controller failure is something to think about. Hardware RAID is still 
going to offer better performance than software RAID (not that software 
is bad, though), but if the controller itself fails, that presents a 
problem you don't have when running software RAID, and this problem 
also presents itself if you want to upgrade controllers (vendor 
lock-in).

If a controller fails and you don't have a spare, you need have very 
current backups so that you can restore to a non-RAID disk and get back 
in action. If you want to change vendors, you need to go that route as 
well. Or if it's really mission critical, build a duplicate box and 
mirror the data over.

With software RAID, you don't have this problem, and you also get a 
performance upgrade anytime you upgrade your CPU.

My recommendation, then, like yours, would be that unless it's a very 
disk-intensive application and the absolute best performance is needed, 
go with software RAID. If you do need the absolute greatest performance 
and reliability, I'd recommend hardware accelerated SCSI RAID. In that 
case, when you choose a controller, buy two identical models. Install 
one, build the RAID array, put a little data on it, then shut down the 
machine and swap in the spare RAID card. Boot up, verify that 
everything still works, you can access the data and it's identical. If 
everything's OK, then put the first card back in the original packaging 
and put it in a safe place in case the running one ever fails.

Jonathan