RE: Storage Cache - WriteThrough or WriteBack

As long as your cache is protected somehow, whether its RAIDed (a la EMC) or mirrored (a la Hitachi), the vast majority of risk associated with write-back cache is mitigated. Even with protected cache, I know of a variety of failure scenarios that will result in loss of in-cache data, but they definitely fall into the "cascading failure", aka "Act of God", category of outages.

Some arrays actually don't even give you the option of write-through cache - on the symmetrix, for example, it is actually impossible for a write to go directly to disk. You have no choice but to cache writes. This is called, in EMC marketing parlance, a "Fast Write". When the cache is under pressure and the symm decides it needs to make more room in cache for an incoming write, it holds the write at the host port, flushes an in-cache write to disk, then places the incoming write in cache and acknowledges it to the host. This is a "Delayed Fast Write" - I love marketing talk. :)

Thanks,
Matt

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Matthew Zito
GridApp Systems
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> -----Original Message-----

> From: ml-errors_at_fatcity.com [mailto:ml-errors_at_fatcity.com] On 

> Behalf Of Jesse, Rich

> Sent: Thursday, August 14, 2003 11:49 AM

> To: Multiple recipients of list ORACLE-L

> Subject: RE: Storage Cache - WriteThrough or WriteBack

> 

> 

> 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

> rjesse_at_qtiworld.com                  Quad/Tech Inc, Sussex, WI USA

> 

> 

> > -----Original Message-----

> > From: Tanel Poder [mailto:tanel.poder.003_at_mail.ee]

> > 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" <ORACLE-L_at_fatcity.com>

> > 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

> > > http://hkchital.tripod.com

> -- 

> Please see the official ORACLE-L FAQ: http://www.orafaq.net

> -- 

> Author: Jesse, Rich

>   INET: Rich.Jesse_at_qtiworld.com

> 

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