8.2-RC1 and ZFS

[David Samms]

On 12/30/10 01:57, Marco van Tol wrote:
> On Wed, Dec 29, 2010 at 05:11:32PM -0700, David Samms wrote:
>> I gave ZFS a try again this afternoon and immediately came across what I
>> would consider a problem.  Simply running dd causes such IO contention
>> that a GUI desktop is unusable and MP3s stop playing.  The problem is
>> very repeatable, just run dd if=/dev/zero of=big bs=1m count=6400
>>
>> System is amd64, source updated Dec 27.  Default ZFS parameters. (no
>> tuning).  With the same system but UFS+s dd causes no problems.
>
> Check out this mail from Jeremy Chadwick from December 28th in
> freebsd-stable at .
> http://docs.freebsd.org/cgi/getmsg.cgi?fetch=312384+0+current/freebsd-stable
>
> I think specifically the comments and tweaks regarding
> vfs.zfs.txg.timeout="5" and vfs.zfs.txg.write_limit_override=1073741824
> should be usefull for you. :-)
>
> I'm intentionally not touching the subject why you see what you do
> without tuning.  I'm sure there are a great bunch of different
> situations for which the defaults are the way they are.
>
> :-)
>
> Marco van Tol
>

Marco, thank you for your reply.  Jeremy, thank you for your tuning 
suggestions.  It does appear that the default setting for ZFS are NOT 
well suited for a single drive setup.  Based on Jeremy's suggestions I 
made the following changes from the defaults:

vfs.zfs.txg.timeout="5"
vfs.zfs.txg.write_limit_override=268435456  # (System has 4G of RAM)
kern.maxvnodes=250000

I tested both UFS+s and ZFS for desktop usability while running "dd" to 
create IO contention.  System has two identical 300G WD drives that are 
about 4 years old.  For testing I switch at boot between the drives, one 
is ZFS the other UFS+s.  Test was performed using XFCE4 with xmms 
playing MP3s while "dd" was creating a large file and OpenOffice was 
launching.

ZFS
----------------------------------------------------------------------
%dd if=/dev/zero of=big bs=1m count=6400
6710886400 bytes transferred in 177.418146 secs (37825254 bytes/sec)

openoffice startup about 185 sec
No MP3 skips
XFCE4 popup menus took 1-3 seconds to appear

UFS+s
----------------------------------------------------------------------
dd if=/dev/zero of=big bs=1m count=6400
6710886400 bytes transferred in 165.061623 secs (40656855 bytes/sec)

openoffice startup about 185 sec
No MP3 skips
XFCE4 popup menus took 5-25 seconds to appear

While dd IO was slower with the new ZFS parameters, the desktop was very 
usable, and its responsiveness was better than USF+s.  I will be 
upgrading to ZFSv28 and repeating this rudimentary desktop usability test.