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

Eberhard Moenkeberg emoenke at gwdg.de
Wed Feb 10 19:06:25 CST 2010


Hi,

On Thu, 11 Feb 2010, Adam Nielsen wrote:

> 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 :-)

Simple: because sudden total power loss could waste the buffer cache.
The battery backuped controller cache would keep the data and write back 
after the next power-on (if it happens within the battery lifetime).


Viele Gruesse
Eberhard Moenkeberg (emoenke at gwdg.de, em at kki.org)

-- 
Eberhard Moenkeberg
Arbeitsgruppe IT-Infrastruktur
E-Mail: emoenke at gwdg.de      Tel.: +49 (0)551 201-1551
-------------------------------------------------------------------------
Gesellschaft fuer wissenschaftliche Datenverarbeitung mbH Goettingen (GWDG)
Am Fassberg 11, 37077 Goettingen
URL:    http://www.gwdg.de             E-Mail: gwdg at gwdg.de
Tel.:   +49 (0)551 201-1510            Fax:    +49 (0)551 201-2150
Geschaeftsfuehrer:           Prof. Dr. Bernhard Neumair
Aufsichtsratsvorsitzender: Dipl.-Kfm. Markus Hoppe
Sitz der Gesellschaft:     Goettingen
Registergericht:           Goettingen  Handelsregister-Nr. B 598
-------------------------------------------------------------------------



More information about the Linux-PowerEdge mailing list