hi folks...
I've just been exposed to zfs directly, since I'm trying it out on
"a certain 48-drive box with 4 cpus" :-)
I read in the archives, the recent " hard drive write cache "
thread. in which someone at sun made the claim that zfs takes advantage of
the disk write cache, selectively enabling it and disabling it.
However, that does not seem to be at all true, on the system I am testing
on. (or if it doesnt, it isnt doing it in any kind of effective way)
SunOS test-t[xxxxxx](ahem) 5.11 snv_33 i86pc i386 i86pc
On the following RAIDZ pool:
# zpool status rzpool
pool: rzpool
state: ONLINE
scrub: none requested
config:
NAME STATE READ WRITE CKSUM
rzpool ONLINE 0 0 0
raidz ONLINE 0 0 0
c0t4d0 ONLINE 0 0 0
c0t5d0 ONLINE 0 0 0
c1t4d0 ONLINE 0 0 0
c1t5d0 ONLINE 0 0 0
c5t4d0 ONLINE 0 0 0
c5t5d0 ONLINE 0 0 0
c9t4d0 ONLINE 0 0 0
c9t5d0 ONLINE 0 0 0
c10t4d0 ONLINE 0 0 0
c10t5d0 ONLINE 0 0 0
Write performance for large files appears to top out at around 15-20MB/sec,
according to zpool iostat
However, when I manually enable write cache on all the drives involved...
performance for the pathalogical case of
dd if=/dev/zero of=/rzpool/testfile bs=128k
jumps to be 40-60MB/sec (with an initial spike to 80MB/sec. i was very
disappointed to see that was not sustained ;-) ]
This kind of performance differential also shows up with "real" load;
doing a tar| tar copy of large video files over NFS to the filesystem.
As a comparison, a single disk's dd write performance is around 6MB/sec no
cache, and 30MB/sec with write cache enabled.
So the 40-50MB/sec result is kind of disappointing, with a **10** disk pool.
Comments?
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