Serious zfs slowdown when mixed with another file system
ricky at csua.berkeley.edu
Sun Jul 11 18:44:14 UTC 2010
This is on clean FreeBSD 8.1 RC2, amd64, with 4GB memory.
The closest I found by Googling was this:
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
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?
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