On 1/19/2010 6:21 AM, Joe Landman wrote:
Rarely. Fake raid will generally not have any RAM cache or battery backup capability.
Not only that, but it won't have any hardware to do parity calculations. (It might be hard to recognize such hardware).
In some instances, fake raid is *ok* for OS drives (RAID1 only), if the bios is smart enough to use it correctly, the underlying fake raid driver is relatively stable, and you have reasonable disks.
It doesn't take much extra work to do RAID0 or RAID1 so whether this is done by a fake raid driver or the md raid driver probably isn't significant from the resource usage point of view. The only advantage I can think of for fake raid is that there's usually a BIOS of sorts in the fake raid card that lets you manipulate the raid units. This might be more convenient than having to boot Linux and mess with mdadm commands. For RAID levels that require parity calculations, then having a hardware RAID card is a win because the card does a lot of work and hides both the parity calculations and required IOs from the host system. On the third hand, if you have a system with lots of CPU and I/O capacity that wouldn't otherwise get used, then it could be argued that a hardware RAID card is an unnecessary expense. In the old days it was easier to decide to go with hardware RAID. These days it's best to do test with both hardware and software RAID, and then see if the measured improvements of hardware RAID (if any) justify its expense. Of course, in any production system you'll want a few extra RAID cards lying around just in case. Cordially, -- Jon Forrest Research Computing Support College of Chemistry 173 Tan Hall University of California Berkeley Berkeley, CA 94720-1460 510-643-1032 jlforr...@berkeley.edu _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
