Re: SAN performance mystery

"Alex Turner" <armtuk@gmail.com> writes:

> Given the fact that most SATA drives have only an 8MB cache, and your RAID
> controller should have at least 64MB, I would argue that the system with the
> RAID controller should always be faster.  If it's not, you're getting
> short-changed somewhere, which is typical on linux, because the drivers just
> aren't there for a great many controllers that are out there.

Alternatively Linux is using the 1-4 gigabytes of cache available to it
effectively enough that the 64 megabytes of mostly duplicated cache just isn't
especially helpful...

I never understood why disk caches on the order of megabytes are exciting. Why
should disk manufacturers be any better about cache management than OS
authors?

In the case of RAID 5 this could actually work against you since the RAID
controller can _only_ use its cache to find parity blocks when writing.
Software raid can use all of the OS's disk cache to that end.

--
greg