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RE: Storage Cache - WriteThrough or WriteBack

From: Jesse, Rich <>
Date: Thu, 14 Aug 2003 07:49:29 -0800
Message-ID: <>

Like any good DBA/SA should say "It depends." WriteBack gives you better write performance since the IO only needs to hit the cache to report back as being completed, whereas WriteThru needs to verify the write hit the disk first. Either should give the same performance on reads, provided the cache isn't the point of contention because of heavy writes.

For our SAN (if we ever get approval for it), we'll probably go with WriteBack. The safety factor will be that the cache will be mirrored and battery-backed, like you mentioned. It's not failsafe (firmware error could conceivably corrupt the mirror, too), but I feel that we'd be hitting major diminishing returns by going farther than that. You'll have to decide what's best for your situation.

BTW, after having someone accidentally kick the power cord out of our existing external storage during a server room rehaul, I'm going to make sure that we have a copy of the control files on a local drive!

HTH! GL! :) Rich

Rich Jesse                           System/Database Administrator                  Quad/Tech Inc, Sussex, WI USA

> -----Original Message-----
> From: Tanel Poder []
> Sent: Thursday, August 14, 2003 3:54 AM
> To: Multiple recipients of list ORACLE-L
> Subject: Re: Storage Cache - WriteThrough or WriteBack
> Hi!
> I usually only tolerate write caching on storage subsystems
> when we are
> dealing with expensive boxes like EMCs Clariion or Symmetrix.
> I too have
> seen caches fail on entry level boxes like Sun A1000 etc...
> Tanel.
> ----- Original Message -----
> To: "Multiple recipients of list ORACLE-L" <>
> Sent: Thursday, August 14, 2003 8:39 AM
> >
> > I've begun a debate in my organisation about
> > caches on storage systems.
> > If an Oracle Database, including Redo Log files,
> > is on RAID1 or RAID1+0 or RAID5 on the storage/SAN
> > and the storage/SAN system provides a cache, should
> > the cache be WriteThrough or WriteBack ?
> >
> > I prefer WriteThrough -- particularly when the
> > Redo Log files are also on such external storage.
> >
> > The vendor talks of Mirrored-Caches and Battery-Backed Cache.
> >
> > In the past year, we've had one instance of the
> > Cache itself failing and the Controller stopping all
> > I/O to the storage and a couple of instances of
> > Cache batteries being low/dead. {Should I/O be
> > allowed to proceed if the Cache Batteries are dead
> > or should the storage automatically switch to WriteThrough ?}
> >
> >
> > Hemant K Chitale
> >

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Author: Jesse, Rich

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Received on Thu Aug 14 2003 - 10:49:29 CDT

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