RE: Slow AS400 performance

["Christen, Duane J." <dchristen@xxxxxxxxxxxxx>]

When data gets written to the cache it is considered "written to disk" even
though it has not been written to disk yet. When main power is lost the data
in the cache is protected by the battery for several days (weeks?). When
main power is restored the controller will write the data to disk and you
won't have lost a thing.
If for some reason the data in the cache is lost you could potentially have
to do a full restore on the system which for our main production system
could take 24-48 hours. (I don't know if RAID stripes are stored in the
cache before being written to disk, but that would make a valid cache even
more critical)

IBM has determined that at some point in the life of the battery it will not
be able to protect the cache for the desired length of time without main
power, and warns you in plenty of time to replace it. If the battery is not
replaced in time the cache is shut off and everything is forced to disk when
it reaches the controller causing the performance degradation.


Duane Christen

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx]On Behalf Of Colin Williams
Sent: Thursday, April 20, 2006 3:25 PM
To: Midrange Systems Technical Discussion
Subject: Re: Slow AS400 performance


I have no idea about the design of these controllers,

but presumably if they are there as battery backup in the event of a power
failure, to ensure that all writes go to disk,

why do they affect performance if they die whilst mains power is available?
Why cant these controllers just use mains power whilst its available?




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