RAID battery-backed cache - necessary? (was: Linux-PowerEdge Digest, Vol 68, Issue 35)

Mann, Andrew amann at ea.com
Wed Feb 10 18:42:09 CST 2010


	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-bounces at dell.com [mailto:linux-poweredge-bounces at dell.com] On Behalf Of Adam Nielsen
Sent: Wednesday, February 10, 2010 7:32 PM
To: linux-poweredge at 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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