Date: Fri, 16 Sep 2011 17:39:56 -0400 From: freebsd@top-consulting.net To: freebsd-questions@freebsd.org Subject: Re: FS of choice for max random iops ( Maildir ) Message-ID: <20110916173956.18133qtvqi91m3wg@mail.top-consulting.net> In-Reply-To: <D0FD7882-E1F6-45C4-B3BE-58E046401699@elde.net> References: <20110916063153.200375qdq59crf8c@mail.top-consulting.net> <32990703-D068-4B0D-AF3A-C1E6EA0A4100@elde.net> <20110916101833.17485ybnq5srjbc4@mail.top-consulting.net> <D0FD7882-E1F6-45C4-B3BE-58E046401699@elde.net>
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Quoting Terje Elde <terje@elde.net>: > On 16. sep. 2011, at 16:18, freebsd@top-consulting.net wrote: > >> Got a measly 74MB/sec. > > You can't ask for advice, get it, do something completely different, > and then complain that it didn't work. > > Neither can you ask people to donate their time, if you won't spend yours. > > In other words: if you won't listen, there's no point in us talking. > > However: > > Don't disable ZIL. Just don't. It's not the way to go. If you want > to know why, google will help. > > Also, you're making some assumptions, such as the ZIL being bad for > performance. That's not always the case. ZIL-writes are a rather > nice load for spinning metal storage. Even if you write through > cache, that can give you a boost on your real world workload. > > Which brings us to the third bit. You're benchmarking, not trying > real world loads. That's the load you'll have to worry about, and > it's the load zfs shines at. > > Thanks to the ZIL (the thing you're trying to kill, remember?) you > can convert seek heavy writes to sequential zil-writes, freeing up > disk bandwith for concurrent reads. > > If you want to test before spending money, try what Svein said. Set > up a small logical volume (preferrably smaller than your controller > cache, if it's large enough), then try that as a dedicated zil-device. > > Never tried that, but worth a shot. > > Terje It's not about spending money or not. I really want to use ZFS for some of its features ( journaled, snapshots, etc ) but it has to be a good fit for me. I'm not ignoring the advice I am given, just taking it with a grain of salt disabling the ZIL is recommended - sometimes - for NFS. As per hundreds of messages I've read from the Archive along with this page, http://wiki.freebsd.org/ZFSTuningGuide, it does appear that disabling the ZIL is a solution for NFS. Yes, they still recommend SSD drives and I fully understand that. My point was the following: Why is a sequential write test like dd slower on ZFS than on UFS ? The writes is already serialized so enabling/disabling the ZIL should have very little impact - which is indeed the case. I even went as far as disabling the cache flush option of ZFS through this variable: vfs.zfs.cache_flush_disable: 1, since I already have the write cache of the controller. I've also set some other variables as per the Tuning guide but according to several benchmarks ( iozone, bonnie++, dd ) ZFS still comes in slower than UFS at pretty much everything. Either I am missing something or there is something wrong with my setup.
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