If the power fails to the system, or the kernel crashes, the OS level
cache will never be written to disk and data may be lost. In the event of a
power failure, the battery backed RAID cache will be written to disk when power
is restored.
This allows processes that like to confirm their data has been stored
to disk before moving on (like databases, or even journaling file system
writes) to continue once the data has been written to the RAID cache - which
completes significantly faster than the physical media can complete the
operation.
Andrew
-----Original Message-----
From: linux-poweredge-boun...@dell.com
[mailto:linux-poweredge-boun...@dell.com] On Behalf Of Adam Nielsen
Sent: Wednesday, February 10, 2010 7:32 PM
To: linux-poweredge@dell.com
Subject: Re: RAID battery-backed cache - necessary? (was: Linux-PowerEdge
Digest, Vol 68, Issue 35)
This is perhaps off-topic too, but I have always wondered...
> You might also want to look at getting a hardware RAID card or
> daughterboard like the PERC-6i - these will allow you to set up a
> RAID-10/50/60 that will stripe all data between two drives, giving you
> another twofold speed increase. You probably want to make sure that your
> card has battery backup if you care about your database - otherwise a
> power cut can lose cached data rather painfully, even if you have a UPS.
> If you're moderately paranoid, or your data is important, you should
> disable on-drive write caching, as these never have battery backup - but
> this will cost you some speed. (This is a software issue, though, and
> won't affect your purchased configuration.)
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?
Anyway, just something I've often wondered about :-)
Cheers,
Adam.
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