battery question

[P.A]

Great reply, thanks.

-----Original Message-----
From: Preston Hagar [mailto:prestonh at gmail.com] 
Sent: Thursday, June 11, 2009 1:39 PM
To: P.A
Cc: linux-poweredge at lists.us.dell.com
Subject: Re: battery question

On Thu, Jun 11, 2009 at 11:57 AM, P.A<razor at meganet.net> wrote:
> Jason I understand about the right caching, but what do you mean about the
> 72 hours unplugged?

The purpose of a battery on a RAID card is so that you can use write
caching.  The way this works is that the OS writes something to the
RAID array, the RAID array stores the write in its memory and then
tells the OS that the write is complete.  Then, when the disks are
ready, the RAID card writes the data to disk.  This allows the OS to
continue working, instead of waiting for the disks to truly complete
the write, increasing performance.  The problem, however, is that if
there is a power failure after the RAID card tells the OS that the
write completed, but before the data is actually written to disk, then
you would likely have a very corrupt, possibly unrecoverable file
system (depending on which file system you used and what was being
written).  To help lessen the risk of this, some RAID cards, like the
PERC 5/i have a on-board battery that will keep the volatile on-board
RAM "alive" with the data for up to 72 hours.  That way, once your
start the machine back up, the first thing the RAID card would do is
flush its cache to disk (completing the writes).

There are a few things to know about this overall.  When the RAID
array was setup, it was setup as either Write Through or Write Back.
If I remember Dell PERC terminology correctly, write back uses the
cache and write through does not.  If you are not using the cache
(writing directly to disk) then the battery doesn't really make a
whole lot of difference.  As a side note, a lot of RAID cards
(especially lower end ones) either have an optional on-board battery,
or no battery option at all.

Secondly, if you have the server on a good reliable UPS, then it isn't
quite as big of a concern, although it is still a possible one.  UPSs
can fail, go out, or not last as long as planned and if you didn't get
the server shut down or at least stop it from writing to the RAID
array before the power went out, then you would have the same issue as
if you had no UPS at all.

Lastly,  the battery on a RAID card is a typical rechargeable battery
and like most of them, will fail to hold a charge after a while.
Since your equipment is out of warranty, I am going to guess that it
is a little bit older and therefore it is more likely that the battery
is worn out.

To answer your original question,  if you reboot properly (as opposed
to having the server go out due to a power outage) then you should not
lose any data.  To fix it, you just need to order a new battery for
your PERC card, power down your server, replace the battery, and power
it back up.

Just for reference, here is a link to the PERC 5/i manual which
includes instructions on replacing a battery on a PERC 5/i

http://support.dell.com/support/edocs/storage/RAID/PERC5/en/UG/HTML/index.ht
m

Hope this helps,

Preston