Date: Thu, 13 Jun 2013 10:28:24 +0200 From: Robert Schulze <rs@bytecamp.net> To: freebsd-fs@freebsd.org Subject: Re: An order of magnitude higher IOPS needed with ZFS than UFS Message-ID: <51B982A8.10605@bytecamp.net> In-Reply-To: <20130612114937.GA13688@icarus.home.lan> References: <51B79023.5020109@fsn.hu> <op.wykdduw834t2sn@markf.office.supranet.net> <20130612114937.GA13688@icarus.home.lan>
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Hi, Am 12.06.2013 13:49, schrieb Jeremy Chadwick: > On Wed, Jun 12, 2013 at 06:40:32AM -0500, Mark Felder wrote: >> On Tue, 11 Jun 2013 16:01:23 -0500, Attila Nagy <bra@fsn.hu> wrote: >> >> ZFS write performance can begin to drop pretty badly when you get >> around 80% full. I've not seen any benchmarks showing an improvement >> with a very fast and large ZIL or tons of memory, but I'd expect >> that would help significantly. Just note that you're right at the >> edge where performance gets impacted. > > Mark, do you have any references for this? I'd love to learn/read more > about this engineering/design aspect (I won't say flaw, I'll just say > aspect) to ZFS, as it's the first I've heard of it. this is even true when getting near a quota limit on a zfs, although there are e.g. 10/16 TB free in the pool. Just create a filesystem and set quota=1G, then do sequential invocations of dd to fill the fs with 100M files. You will see a sharp slowdown when the last twenty files are beeing created. Here are the results from the following short test: for i in `jot - 0 99` do dd if=/dev/zero of=/pool/quota-test/10M.$i bs=1M count=10 done 0..80: < 0.4 s 80 0.27 s 81 0.77 s 82 0.50 s 83 0.51 s 84 0.22 s 85 0.87 s 86 0.52 s 87 1.13 s 88 0.91 s 90 0.39 s 91 1.04 s 92 0.80 s 93 1.94 s 94 1.27 s 95 1.36 s 96 1.76 s 97 2.13 s 98 3.28 s 99 4.07 s of course, there are some small values beyond 80% utilisation, but I think the trend is clearly visible. In my opinion, hitting a quota limit should not give these results unless enough free physical disk space is available in the pool. This is a bug or a design flaw and creating serious problems when exporting quota'ed zfs over nfs. with kind regards, Robert Schulze -- /7\ bytecamp GmbH Geschwister-Scholl-Str. 10, 14776 Brandenburg a.d. Havel HRB15752, Amtsgericht Potsdam, Geschaeftsfuehrer: Bjoern Barnekow, Frank Rosenbaum, Sirko Zidlewitz tel +49 3381 79637-0 werktags 10-12,13-17 Uhr, fax +49 3381 79637-20 mail rs@bytecamp.net, web http://bytecamp.net/
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