PE1850 / PERC 4e/Si / RHEL4 problem

Jon Etkins jetkins at austinlogistics.com
Tue May 9 13:33:16 CDT 2006


Actually, it's a correct assumption at the driver level.  The driver
assumes write-through, which means that any data that it passes to what
it sees as a physical disk (but is in fact the cache-backed RAID
controller) is guaranteed to be written to the disk before the I/O is
completed.  The fact that it may actually reside temporarily in the
battery-backed controller cache before actually being written to the
underlying physical media is immaterial to the driver - it's been
delivered to the disk subsystem and WILL be written to the drive.


Jon Etkins
Network & IT Administrator
Austin Logistics, Incorporated
(512) 651-5641
www.AustinLogistics.com
 


-----Original Message-----
From: linux-poweredge-bounces at dell.com
[mailto:linux-poweredge-bounces at dell.com] On Behalf Of J. Epperson
Sent: Tuesday, May 09, 2006 12:10 PM
To: linux-poweredge at dell.com
Subject: Re: PE1850 / PERC 4e/Si / RHEL4 problem


"Without this information, the block layer assumes "write-through". This
is safe, because the controller has a battery-backed cache.".

Write-through is safe WITHOUT a battery-backed cache--the block layer is
treating you as though you DON'T have one.  The battery-backed cache
protects you in "write-back" mode.

"...it's not an error, and it doesn't affect the system."

Other than delaying your boot by at least a minute, only to make an
erroneous assumption.
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