Date: Wed, 12 Jun 2013 19:54:36 +0200 From: Adam Nowacki <nowakpl@platinum.linux.pl> To: freebsd-fs@freebsd.org Subject: Re: An order of magnitude higher IOPS needed with ZFS than UFS Message-ID: <51B8B5DC.2010703@platinum.linux.pl> 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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On 2013-06-12 13:49, Jeremy Chadwick wrote: > 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: >> >>> BTW, the file systems are 77-78% full according to df (so ZFS >>> holds more, because UFS is -m 8). >> >> 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. > > The reason I ask: (respectfully, not judgementally) I'm worried you > might be referring to something that has to do with SSDs and not ZFS, > specifically SSD wear-levelling performing better with lots of free > space (i.e. a small FTL map; TRIM helps with this immensely) -- where > the performance hit tends to begin around the 70-80% mark. (I can talk > more about that if asked, but want to make sure the two things aren't > being mistaken for one another) > So I went hunting for some evidence and created this: http://tepeserwery.pl/nowak/fillingzfs.png Columns are groups of sectors, new row is created every time a FLUSH command is sent to a disk. Percentage is the amount of filled space in the pool. Red means a write happened there, Pool is 1GB with writes of 50MB between black lines. It looks like past 80% there simply isn't enough continuous disk space and writes are becoming more and more random. For some unknown to me reason there is also a lot more flushing which certainly doesn't help for performance. There is also this odd hole left untouched by any write, reserved space of some sort?
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