[Asterisk-Users] RAID affecting X100P performance...

[Scott Laird]

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