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

Ryan Miller rmiller at smartertravelmedia.com
Wed Feb 10 18:44:24 CST 2010


It's about write caching.  Fsync() can't return (and thus, e.g., your sql transaction can't return committed) until the data is on durable storage (that's what the D in ACID means).  Battery-backed cache on the controller counts, because if you pull the plug, the data will still get to disk.  O/S disk cache in RAM doesn't, for the same reason.  So whether you see substantial performance improvements with battery backed controller cache depends on the read/write ratio of your workload and the frequency of fsync.  Since the cache is small, it's most helpful for small random writes with fsync, punctuated by more idle periods where the cache can spool out to disk.

> -----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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