ATA_CAM + ZFS gives short 1-2 seconds system freeze on disk load

[Jeremy Chadwick]

On Mon, Feb 08, 2010 at 05:58:53PM +0100, Guido Falsi wrote:
> On Mon, Feb 08, 2010 at 05:18:22PM +0100, Fabian Keil wrote:
> > 
> > I experienced what I think is the same problem. ZFS's bulk disk flushes
> > caused vlc to occasionally stutter when viewing a DVD rip from disk while
> > ripping a DVD at the same time.
> > 
> > My workaround is to put "vfs.zfs.txg.timeout=3" in /boot/loader.conf.
> > I think I read about this on zfs-discuss at . I assume on faster systems
> > one can use a higher value.
> 
> This was a very good idea. I'm trying this and the system freezes much
> less if at all.
> 
> Are there any drawbacks I should be aware of?
> 
> I can imagine that forcing more disk writes somewhat slows down normal
> disk activity, but I can bear that on a desktop system.

I'd like a technical explanation of exactly what this loader.conf
tunable does.  The sysctl -d explanation is useful if one has insight to
what purpose it serves.  I can find mention on Solaris lists about "txg
timeout", but the context is over my head (intended for those very
familiar with the inner workings of ZFS).

I'd also like to know what vfs.zfs.txg.synctime does.

I've been playing with vfs.zfs.txg.timeout (decreasing it from 30 to 5)
and testing on a 3-disk raidz1 pool on a test machine I have, but I
haven't finished my testing yet.  A person said that decreasing the
value increased throughput[1] for them, but in my brief test (using dd)
it doesn't appear to improve I/O speed at all.

Anyone know of a good (non-X-related) repro test case which can induce
the ZFS "bursty sluggishness" problem so I can try it with different
loader.conf tunables?

Thanks.

-- 
| 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 |