Serious zfs slowdown when mixed with another file system (ufs/msdosfs/etc.).

[Richard Lee]

On Sun, Jul 11, 2010 at 01:47:57PM -0700, Jeremy Chadwick wrote:
> On Sun, Jul 11, 2010 at 11:25:12AM -0700, Richard Lee wrote:
> > This is on clean FreeBSD 8.1 RC2, amd64, with 4GB memory.
> > 
> > The closest I found by Googling was this:
> > http://forums.freebsd.org/showthread.php?t=9935
> > 
> > And it talks about all kinds of little tweaks, but in the end, the
> > only thing that actually works is the stupid 1-line perl code that
> > forces the kernal to free the memory allocated to (non-zfs) disk
> > cache, which is the "Inact"ive memory in "top."
> > 
> > I have a 4-disk raidz pool, but that's unlikely to matter.
> > 
> > Try to copy large files from non-zfs disk to zfs disk.  FreeBSD will
> > cache the data read from non-zfs disk in memory, and free memory will
> > go down.  This is as expected, obviously.
> > 
> > Once there's very little free memory, one would expect whatever is
> > more important to kick out the cached data (Inact) and make memory
> > available.
> > 
> > But when almost all of the memory is taken by disk cache (of non-zfs
> > file system), ZFS disks start threshing like mad and the write
> > throughput goes down in 1-digit MB/second.
> > 
> > I believe it should be extremely easy to duplicate.  Just plug in a
> > big USB drive formatted in UFS (msdosfs will likely do the same), and
> > copy large files from that USB drive to zfs pool.
> > 
> > Right after clean boot, gstat will show something like 20+MB/s
> > movement from USB device (da*), and occasional bursts of activity on
> > zpool devices at very high rate.  Once free memory is exhausted, zpool
> > devices will change to constant low-speed activity, with disks
> > threshing about constantly.
> > 
> > I tried enabling/disabling prefetch, messing with vnode counts,
> > zfs.vdev.min/max_pending, etc.  The only thing that works is that
> > stupid perl 1-liner (perl -e '$x="x"x1500000000'), which returns the
> > activity to that seen right after a clean boot.  It doesn't last very
> > long, though, as the disk cache again consumes all the memory.
> > 
> > Copying files between zfs devices doesn't seem to affect anything.
> > 
> > I understand zfs subsystem has its own memory/cache management.
> > Can a zfs expert please comment on this?
> > 
> > And is there a way to force the kernel to not cache non-zfs disk data?
> 
> I believe you may be describing two separate issues:
> 
> 1) ZFS using a lot of memory but not freeing it as you expect
> 2) Lack of disk I/O scheduler
> 
> For (1), try this in /boot/loader.conf and reboot:
> 
> # Disable UMA (uma(9)) for ZFS; amd64 was moved to exclusively use UMA
> # on 2010/05/24.
> # http://lists.freebsd.org/pipermail/freebsd-stable/2010-June/057162.html
> vfs.zfs.zio.use_uma="0"
> 
> For (2), may try gsched_rr:
> 
> http://svnweb.freebsd.org/viewvc/base/releng/8.1/sys/geom/sched/README?view=markup
> 
> -- 
> | 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 |

vfs.zfs.zio.use_uma is already 0.  It looks to be the default, as I never
touched it.  And in my case, Wired memory is stable at around 1GB.  It's
the Inact memory that takes off, but only if reading from non-zfs file
system.  Without other file systems, I can keep moving files around and
see no adverse slowdown.  I can also scp huge files from another system
into the zfs machine, and it doesn't affect memory usage (as reported by
top), nor does it affect performance.

As for gsched_rr, I don't believe this is related.  There is only ONE
access to the zfs devices (4 sata drives), which is purely a sequential
write.

The external USB HDD (UFS2) is a completely different device, and is doing
purely sequential read.  There is only one "cp" process doing anything at
all.

The FreeBSD system files aren't on either of these devices, either not
that it's doing anything with the disk during this time.

--rich