RE: Storage Cache - WriteThrough or WriteBack

"Delayed Fast Write"

I would love to hear that in a sales pitch. ;)

Jared

"Matthew Zito" <mzito_at_gridapp.com>

Sent by: ml-errors_at_fatcity.com
 08/14/2003 09:29 AM
 Please respond to ORACLE-L  

        To:     Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
        cc: 
        Subject:        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

--
Matthew Zito
GridApp Systems
Email: mzito_at_gridapp.com
Cell: 646-220-3551
Phone: 212-358-8211 x 359
http://www.gridapp.com




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