On Jul 21, 2004, at 1:33 PM, Steven Critchfield wrote:
Software raid is bad. IDE hardware raid isn't much better. Software raid
is always going to eat your system alive since the CPU has to be busy
with 2 or more writes as opposed to it's normal 1.
That hasn't been my experience at all. Frankly, I've never seen a cheap (<$3k) hardware RAID controller that can touch software RAID's performance on Linux, especially in "challenging" setups, like RAID-5. Sure, software RAID eats more CPU, but most PCs have CPU to spare these days. Would you rather eat 10% of one of your Xeon CPUs to get 200 MB/sec or 100% of an Intel 960 to get 15 MB/sec?
Having said that, booting off of software RAID is a total pain in the neck in Linux. You're *much* better off buying a 3ware RAID card if you want to boot off of RAID-1 IDE drives. Avoid motherboard RAID like the plague--it's almost always just software RAID with a BIOS-level driver and a proprietary disk format.
From a TCO standpoint, the 3ware cards are usually cheaper then the time it takes to configure and manage software RAID in Linux. That'll change some day, but the day's still quite a ways off.
Raid 5 spreads the load over spindles and should take less CPU total, but don't bet on it if it is IDE.
That's exactly backwards--RAID 5 eats more CPU then RAID 0 or 1, because it needs to XOR all of your data to generate parity. With an expensive hardware RAID controller, you'll have a fast enough CPU to handle it all on the card. With a cheaper RAID controller, you'll be massively bottlenecked whenever you're writing to disk. With software RAID, you'll eat more system CPU, but still get good streaming I/O performance. If you're doing small random writes, you'll get horrible performance unless you have a *really* bright RAID controller.
For this (and a number of other reasons), you're best off avoiding RAID 5 if you care about random I/O performance. It can be made to go fast, but you'll need to throw a lot of cash at it. The same amount of cash will frequently get you better performance with RAID 0+1 (or 1+0, depending on how you look at things).
Go SCSI or don't do RAID.
SCSI has its places--it's way faster when you care about lots of small I/O operations, and the drives are somewhat more reliable, but it's massively more expensive, especially if you're more concerned about storage capacity then performance. Personally, I'd take IDE RAID 1 over a single SCSI drive for critical data almost any day of the week. It's an engineering trade-off (the extra complexity of RAID and the higher failure rate of IDE vs the lack of a safety net with a single SCSI drive), but when money isn't growing on trees, it's a nice option to have.
In the context of Asterisk, where disk I/O is either logging or voicemail, buying a 3ware card and a pair of IDE drives seems like a decent business decision.
Scott
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