Disappointing speed with ZFS

Kris Kennaway kris at FreeBSD.org
Mon Feb 11 20:28:25 UTC 2008

Alexey Tarasov wrote:
> I've done similar tests on the other machine, and all looks fine.
> But why on this machine ZFS works slower than UFS? When I make UFS file 
> system on the same disk, rtorrent hashing works 10 times faster. And 
> while hashing, HDD is used three times intensively with ZFS (noticed by 
> flashing LED).
> I have an amd64 Core2Duo processor, 4 Gb of RAM, what is not enough for 
> ZFS?
> What kernel tuning can help me?

I'd guess this is just related to the ZFS design.  As Ivan says, it 
prefers to do all writes sequentially.  This means that reads (as with 
reading of hashes) may be very fragmented and require lots of drive 
seeking, which will reduce performance a lot.  ZFS does do aggressive 
prefetching of data to try and offset this problem, but if your disk 
bandwidth is low (e.g. you are not using a fast disk array) then it may 
not help much (and can also introduce big I/O latency for other operations).

As for what can be done about this, I don't know, but you should look 
into the general ZFS literature (ZFS support mailing lists, etc).


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