Re: ZFS & disk cache - Mailing list pgsql-general

On Mon, 1 Jun 2009, Scara Maccai wrote:

> "The Solaris ZFS file system is safe with disk write-cache enabled because it issues its own disk cache flush
commands"
> Could someone explain?

There are discussion of this with more information at:

http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide#Cache_Flushes
http://milek.blogspot.com/2008/02/2530-array-and-zfs.html
http://blogs.digitar.com/jjww/2006/12/shenanigans-with-zfs-flushing-and-intelligent-arrays/

The only situation I'm aware of where ZFS may not be able to turn off the
unsafe write cache inside a disk is with some of the newer SSD devices.
For example, Jignesh Shah from Sun reported at the last PG East that
Solaris wasn't doing the right thing to disable the write cache inside the
Intel SSD devices he tested, which means they're doing write caching
without a battery that could lead to database corruption.  See page 13 of
the presentation for
http://blogs.sun.com/jkshah/entry/effects_of_flash_ssd_on for his quick
note on that topic.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD