zfs and BIO_FLUSH on amr(4)

[Pawel Jakub Dawidek]

On Tue, Aug 28, 2007 at 03:00:44AM -0700, Simun Mikecin wrote:
> Using storage on amr(4) controller with zfs revealed reduced write performance.
> It seems that writing to zfs concludes with a BIO_FLUSH call to flush the controller cache to
> disk.
> 
> amr(4) can have onboard cache memory with or without battery backup. Is BIO_FLUSH call really
> needed when amr(4) is used with onboard cache memory with a battery backup?

I don't think so. The thing is that when ZFS receives information that
write is done, it should be on disk (at some point) and you
battery-backed cache should ensure that. You can turn off sending
BIO_FLUSH by setting vfs.zfs.cache_flush_disable to 1 (in
/boot/loader.conf).

BTW. How big performance drop do you see with BIO_FLUSH turned on?

-- 
Pawel Jakub Dawidek                       http://www.wheel.pl
pjd at FreeBSD.org                           http://www.FreeBSD.org
FreeBSD committer                         Am I Evil? Yes, I Am!
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