zfs and BIO_FLUSH on amr(4)

[Simun Mikecin]

--- Pawel Jakub Dawidek <pjd at FreeBSD.org> wrote:
> On Tue, Aug 28, 2007 at 03:00:44AM -0700, Simun Mikecin wrote:
> > 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?

Not much according to bonnie++:

Results from running "bonnie++ -d /var/tmp/test -s 4g -n 100 -u someuser:somegroup"

With vfs.zfs.cache_flush_disable=0:

Version 1.93c       ------Sequential Output------ --Sequential Input- --Random-
Concurrency   1     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
data.home.hr     4G   105  97 57927  15 44814  11   261  97 150219  17 281.0 4
Latency               147ms    1669ms     964ms   83179us     551ms     241ms
Version 1.93c       ------Sequential Create------ --------Random Create--------
data.home.hr        -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                100 19034  92 28638  98 10672  91 13761  98 32042  97 14009 95
Latency             77131us    2460us    3298us     153ms    7728us3796us


With vfs.zfs.cache_flush_disable=1:

Version 1.93c       ------Sequential Output------ --Sequential Input- --Random-
Concurrency   1     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
data.home.hr     4G   118  94 67671  16 41687  10   292  99 154120  20 290.1   4
Latency               764ms    1743ms    1586ms   78276us     342ms     245ms
Version 1.93c       ------Sequential Create------ --------Random Create--------
data.home.hr        -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                100 22064  91 33277  97 13824  93 16720  86 23878  99 10182  95
Latency             68420us    2832us    3789us     727ms    7670us    3838us



       
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