Philip Brown writes:
> 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 ;-) ]
Yes it is; See "Sequential writing is jumping"; should not
be too hard to fix though.
http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6415647
>
> 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.
>
I Don't think RAID-Z is your problem in the above, but if the
performance of random read is important do check this:
http://blogs.sun.com/roller/page/roch?entry=when_to_and_not_to
-r
>
>
> Comments?
>
>
>
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