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Date: Wed, 17 Nov 2010 09:09:44 +0100 From: Rogier Wolff <R.E.Wolff@...Wizard.nl> To: Pavel Machek <pavel@....cz> Cc: linux-kernel@...r.kernel.org Subject: Re: Sync semantics. On Tue, Nov 16, 2010 at 03:31:49PM +0100, Pavel Machek wrote: > > I would expect that all buffers that are dirty at the time of the > > "sync" call are written by the time that sync returns. I'm currently > > bombarding my fileserver with some 40-60Mbytes per second of data to > > be written (*). The fileserver has 8G of memory. So max 8000 Mb of > > Are you sure? Hitting 40MB/sec is hard when it involves seeking... Yeah... It's about 10 times slower than when no seeking is involved, so that makes sense, doesn't it? The machine can sustain over 400 Mb per second on linear reads: procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 2 0 0 50908 6667292 502040 0 0 430064 0 2171 1677 0 23 66 11 4 0 0 51280 6713952 501976 0 0 429596 0 2430 1889 16 28 44 12 1 0 0 51768 6754884 502100 0 0 423388 0 2460 2100 13 28 47 13 0 1 0 50760 6793392 502416 0 0 422892 0 2174 1796 0 21 68 10 Through the filesystem I get: 1073741824 bytes (1.1 GB) copied, 2.70151 s, 397 MB/s 1073741824 bytes (1.1 GB) copied, 2.62782 s, 409 MB/s Which impresses me. In practise I seldomly see high 1xxMb/sec. (i.e. 120-150Mb per second happens, while 180-190 is rare). On the other hand, in the same run I also get: 1073741824 bytes (1.1 GB) copied, 6.82678 s, 157 MB/s 1073741824 bytes (1.1 GB) copied, 6.66133 s, 161 MB/s 1073741824 bytes (1.1 GB) copied, 6.58995 s, 163 MB/s which apparently is caused by these files being more fragmented. These files (1Gb each) were written linearly, but some might have been written wile other of these 1G files (in a different directory) were written at the same time. I'm guessing these ended up more or less interleaved. Checking up on the fragmentation of these files, the fast ones have about 600-800 fragments, while the slow ones have 1300-2000 fragments. Mb/sec #frags 400 1252 493 865 391 755 393 606 395 819 206 937 159 901 173 1940 165 1806 157 1481 168 1351 179 2692 166 1541 154 1151 159 924 149 1228 155 1139 151 1103 150 1070 155 1160 There is SOME correlation but not 100%. This is on an 8x1T RAID. > You may want to lower dirty_ratio... You know, what I would REALLY want is that when say 400Mb of dirty buffers exist, the machine would start alternating between the two or three areas that require writing. All these should be "linear". If you switch only once every second or so, the "seeking time" is less than 1%. In that case, my server should be able to write up to 400Mb per second, except for that I can only supply 120Mb per second over the Ethernet. But that would still be a 3x improvement over what the machine can handle now. In theory these things should work even better if things like "dirty_ratio" are higher. In the current situation, the "sync" call will return when the IO system falls to "idle". The chances of "nothing needing writing" increases as the amount of allowed buffers is lower. But the problem is that sync keeps on waiting for those new "dirty" buffers that have become dirty AFTER the start of the sync call. Suppose we have a mail handling daemon that just recieved an Email from over the network. Instead of just saying: Ok, i'll take over from here, it prefers to write it to disk, and calls sync, so that should the power fail, the EMail is on permanent storage, and can be correctly handled. This works just great, until someone manages to get the server to continue to get new dirty buffers, so that the sync takes over ten minutes, and the other sides MTA will time out..... Anyway, someone told me that it's been fixed, and sync won't behave like this anymore. Roger. > -- > (english) http://www.livejournal.com/~pavelmachek > (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html > -- ** R.E.Wolff@...Wizard.nl ** http://www.BitWizard.nl/ ** +31-15-2600998 ** ** Delftechpark 26 2628 XH Delft, The Netherlands. KVK: 27239233 ** *-- BitWizard writes Linux device drivers for any device you may have! --* Q: It doesn't work. A: Look buddy, doesn't work is an ambiguous statement. Does it sit on the couch all day? Is it unemployed? Please be specific! Define 'it' and what it isn't doing. --------- Adapted from lxrbot FAQ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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