> Is there a specific reason why you need to do the caching at the DB > level instead of the file system? I'm really curious as i've got > conflicting data on why people do this. If i get more data on real > reasons on why we shouldn't cache at the file system, then this could > get bumped up in my priority queue.
FWIW a MySQL database was recently moved to a FreeBSD system with ZFS. Performance ended up sucking because for some reason data did not make it into the cache in a predictable fashion (simple case of repeated queries were not cached; so for example a very common query, even when executed repeatedly on an idle system, would take more than 1 minute instead of 0.10 seconds or so when cached). Ended up convincing the person running the DB to switch from MyISAM (which does not seem to support DB level caching, other than of indexes) to InnoDB, thus allowing use of the InnoDB buffer cache. I don't know why it wasn't cached by ZFS/ARC to begin with (the size of the ARC cache was definitely large enough - ~ 800 MB, and I know the working set for this query was below 300 MB). Perhaps it has to do with ARC trying to be smart and avoiding flushing the cache with useless data? I am not read up on the details of the ARC. But in this particular case it was clear that a simple LRU had been much more useful - unless there was some other problem related to my setup or FreeBSD integration that somehow broke proper caching. -- / Peter Schuller PGP userID: 0xE9758B7D or 'Peter Schuller <[EMAIL PROTECTED]>' Key retrieval: Send an E-Mail to [EMAIL PROTECTED] E-Mail: [EMAIL PROTECTED] Web: http://www.scode.org
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