zfs on 4k sector disks

[Jeremy Chadwick]

On Mon, Jul 12, 2010 at 12:25:02PM +0000, S M wrote:
> 
> I found that playing with either:
> 
> (sysctl) vfs.zfs.txg.write_limit_override
> 
> or
> 
> (boot/loader.conf) vfs.zfs.txg.timeout=5
> (boot/loader.conf) vfs.zfs.txg.synctime=5
> 
> Helped with the write problem. What seems to happen is that zfs' calculations go slightly wrong if you have a lot of RAM and therefore zfs buffers too much, forcing so many writes that you lose all read IO for a few seconds....

This has little to do with 4KB sector disks, as it applies to 512-byte
sector disks too.  Some references:

http://lists.freebsd.org/pipermail/freebsd-fs/2009-December/007343.html
http://lists.freebsd.org/pipermail/freebsd-fs/2009-December/007355.html

The values chosen (in your case, "5") greatly depend on the model and
type of hard disk used.  I stress the word "greatly".  Others reading
this mail should take that into consideration.

The default values (meaning out-of-the-box) for these tunables is as
follows (taken from RELENG_8's source as of a few minutes ago).  I'm
also including what source files were used to verify said claims.  For a
description of what these do, see sysctl -d.

vfs.zfs.txg.synctime = 5
  - src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dsl_pool.c
  - src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/txg.c

vfs.zfs.txg.timeout = 30
  - src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/txg.c

vfs.zfs.txg.write_limit_override = 0
  - src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dsl_pool.c
  - src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/txg.c

The defaults may vary per person, based on what source branch/tag they
use (ex. RELENG_8 vs. RELENG_8_0 vs. RELENG_7) and what build date your
kernel is (since what's in /usr/src might not match the running kernel).

To see if these values have changed over time (e.g. in an older release
they may have been different), one should refer to cvsweb or equivalent.

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |