I don't believe ZFS toggles write cache on disks on the fly. Rather,
write caching is enabled on disks which support this functionality.
Then at appropriate points in the code ioctl is called to flush the
cache thereby providing the appropriate data guarantees.
However this by no means addresses your primary issue, viz. the
performance using ZFS on your system is bad.
-Sanjay
Philip Brown wrote:
I previously wrote about my scepticism on the claims that zfs
selectively enables and disables write cache, to improve throughput
over the usual solaris defaults prior to this point.
I posted my observations that this did not seem to be happening in any
meaningful way, for my zfs, on build nv33.
I was told, "oh you just need the more modern drivers".
Well, I'm now running S10u2, with
SUNWzfsr 11.10.0,REV=2006.05.18.01.46
I dont see much of a difference.
By default, iostat shows the disks grinding along at 10MB/sec during
the transfer.
However, if I manually enable write_cache on the drives (SATA drives,
FWIW), the drive throughput zips up to 30MB/sec during the transfer.
Test case:
# zpool status philpool
pool: philpool
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
philpool ONLINE 0 0 0
c5t1d0 ONLINE 0 0 0
c5t4d0 ONLINE 0 0 0
c5t5d0 ONLINE 0 0 0
# dd if=/dev/zero of=/philpool/testfile bs=256k count=10000
# [run iostat]
The wall clock time for the i/o to quiesce, is as espected. Without
write cache manually enabled, it takes 3 times as long to finish, as
with it enabled. (1:30, vs 30sec)
[Approximately a 2 gig file is generated. A side note of interest to
me is that in both cases, the dd returns to the user relatively
quickly, but the write goes on for quite a long time in the
background.. without apparently reserving 2 gigabytes of extra kernel
memory according to swap -s ]
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