Re: Storage Cache - WriteThrough or WriteBack

Hi!

I wonder do you have a fast link to drop about RAIDedness of EMC storage cache?

Tanel.

• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Thursday, August 14, 2003 7:29 PM

>
> 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
>
> --
> Matthew Zito
> GridApp Systems
> Email: mzito_at_gridapp.com
> Cell: 646-220-3551
> Phone: 212-358-8211 x 359
> http://www.gridapp.com
>
> > -----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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> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Matthew Zito
> INET: mzito_at_gridapp.com
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Tanel Poder
  INET: tanel.poder.003_at_mail.ee

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