R. Scott Belford wrote:
of course hardware raids with
battery backup can work around this with lots of cache and keeping the
disks up to date minus cache).
Help me with this one. I built a server with a hardware raid controller. I
noticed that Adaptec offers a backup battery module for it, but I don't
understand the scenario when it would be used. If the card can't get power
from the motherboard, then there is likely no power to the drives. I know I
don't understand something here, can you enlighten me. :-)
scott
In order for the array to be redundant, all drives must be consistant
with each other. This means that the raid controller/software can't
continue with the next operation until ALL devices have completed the
previous one (SCSI can queue up multiple operations, but the write isn't
done until ALL drives in the array are at the same state). In order to
compensate for this, many big RAID cards (like my AMI MegaRAID) have a
battery backed RAM cache.
If the power goes out, all the drives are in a consistant state (as
assured by the controller) with each other, but not nesecarilly with
what the OS thought it was. The cache has to be maintained across
resets to keep the array in a consistant state if cache is to be used
(and believe me, it helps performance tremendously). This is why the
cards have a battery. DRAM (and some cards use SRAM) takes VERY little
power, so a tiny little battery pack can keep it powered for a very long
time (days, months, etc), ensuring that the cache data isn't lost in the
event of an unclean power down. This is always a good idea.
--MonMotha
