RAID battery-backed cache - necessary?

Blake Hudson blake at ispn.net
Sat Mar 6 01:23:54 CST 2010



Adam Nielsen wrote:
>
> I am curious as to why this type of battery-backed cache is important. 
> The OS would do a large amount of caching (Linux can have a disk cache 
> of many gigabytes) which I am sure would be far more effective than the 
> small caches on many RAID cards.
>
> Given that the OS, if configured properly, should provide the best type 
> of caching possible, why is it still necessary to have RAID cache and 
> on-drive cache?  Surely these would provide no additional benefit?
>
>   

While the other posts are accurate, one thing wasn't clarified. And that
is you're confusing read cache and write cache. The "several gigabytes"
are file that linux has *read *into memory from disk - allowing quicker
access if it needs them again. Most linux file systems maintain a small
write cache and flush it to disk every few seconds. This ensures file
system consistency in the event of a crash or power loss.

As the other posters have mentioned, the cache on the RAID cards is
there to improve write performance by making up for (or hiding) the
mechanical limitations of disks. To give you an example, an application
that requires a sync after every operation (like ISC's DHCP server
offering a lease) could only give out a theoretical maximum (actual
results were about 50%) of 250 leases per second (this is the number of
revolutions of a 15k RPM disk drive). By enabling the write cache on a
PERC controller, ISC's DHCP server was able to give out between 1000 and
2000 leases per second (until it became CPU limited) in my testing.


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