Disappointing speed with ZFS

Alexey Tarasov me at lexasoft.ru
Mon Feb 11 17:05:21 UTC 2008

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?

On 11.02.2008, at 17:38, Ivan Voras wrote:

> Alexey Tarasov wrote:
>> Hello.
>> I am trying to use ZFS to store my torrent downloads. I noticed that
>> hashing in rtorrent works 10 times slower than the same disk with  
>> UFS.
> I've done some extensive file system testing and here are my results
> with bonnie++ for UFS+SU vs ZFS on AMD64, 6 GB RAM (1 GB for kmem),  
> on a
> RAID10 volume of 15 kRPM SAS drives:
> UFS+SU: write: 109 MB/s, read: 111 MB/s, random file creation: 36500  
> f/s
> ZFS: write: 95 MB/s, read: 180 MB/s (!!), random file creation:  
> 40522 f/s
> Read speed for ZFS seems too high to be valid, it's probably some  
> cache
> effects (though tests were done on a file more than twice the RAM  
> size).
> In any case, ordinary hashing should cause sequential reading, and  
> these
> seem really fast.
> There could be one more thing: ZFS tries to write data sequentially,
> like a log file system, and if the download was done in "parallel",  
> many
> pieces from different areas of the file at the same time (which is
> normally the case for torrents), it might have gotten very  
> fragmented on
> the drive.
> You can verify this by creating a similarily-sized ordinary file  
> with dd
> (the file should be large enough not to fit in the memory cache, or  
> the
> test should be done after a reboot) and then run iostat in one console
> while reading the files (separately, one at a time, with dd or cat) in
> another. A very fragmented file should have significantly higher tps  
> count.
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Alexey Tarasov

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