Hi Philipp, thanks, if that explains it reasonably well, I will just paraphrase. But first I have to understand it myself ;-) Arno On Tue, Feb 05, 2013 at 10:09:36PM +0100, Philipp Wendler wrote: > Hi, > > Am 05.02.2013 21:45, schrieb Arno Wagner: > > thanks for the info. I think I may add a "performance" > > section to the FAQ, and this could be one of the items. > > Do you have the reference where you found the info on the > > stripe_cache_size? > > Actually, I first read about the concept of a stripe cache in this btrfs > announcement post: > http://article.gmane.org/gmane.comp.file-systems.btrfs/23006 > Then I googled it and found some usages: > http://h3x.no/2011/07/09/tuning-ubuntu-mdadm-raid56 > http://ubuntuforums.org/showthread.php?t=1494846 > > Unfortunately, I didn't find a real citable source. > The kernel documentation (Documentation/md.txt) also only contains 3 > lines saying nothing about what it actually is. > > Greetings, Philipp > > > > On Tue, Feb 05, 2013 at 09:02:21PM +0100, Philipp Wendler wrote: > >> Hello, > >> > >> Am 05.07.2011 08:45, schrieb Philipp Wendler: > >>> I have set up a Linux software RAID5 on three hard drives and want to > >>> encrypt it with cryptsetup/LUKS. My tests showed that the encryption > >>> leads to a massive performance decrease that I cannot explain. > >>> > >>> The RAID5 is able to write 187 MB/s [1] without encryption. With > >>> encryption on top of it, write speed is down to about 40 MB/s. > >> > >> Sorry for answering to a mail in this very old thread, > >> but it seems I finally found a solution, > >> and I know that there are some other people interested in the solution > >> as well (so if you got this email directly from me, I BCC'ed you because > >> you contacted me about this). > >> If you want to read up the full story, here's the link: > >> http://www.saout.de/pipermail/dm-crypt/2011-July/001773.html > >> > >> Today I read about the stripe_cache_size setting of md RAID, and tried > >> it out. With the default value of 256, the performance is slow as > >> described. With a value of 4096, I get a performance increase from about > >> 40-50 MB/s to 123 MB/s. For values >= 8192, I get 140 MB/s out of it. > >> > >> Background: The stripe cache stores recently written blocks. If data is > >> written continuously, it might happen that during a first write only a > >> part of one stripe is written. This means, the RAID code has to read the > >> complete stripe from disk, update it, and write it completely again. If > >> a second write comes in for another part of the same stripe, all this > >> would have to be done again. Now, if the cache is used and still > >> contains the data written by the first write, the read that was > >> necessary before the second write can be omitted. > >> > >> > >> Now it seems that dm-crypt always writes with small block size to the > >> underlying disk, even when I write with a big block size. > >> Could this be true? > >> Could this perhaps be improved? While I have found a solution for me, > >> this could probably solve performance problems for many people. > >> > >> Furthermore, dm-crypt write is still slower than unencrypted write, > >> although for reads the performance of encrypted and unencrypted are the > >> same. So I guess the small block size still has a performance penalty > >> (probably when first writing to a stripe and it is not yet in cache). > >> > >> My current setup is Ubuntu 12.04 (Linux 3.2). > >> Nothing else has changed compared to when I first asked about this. > >> > >> Greetings, Philipp > >> _______________________________________________ > >> dm-crypt mailing list > >> dm-crypt@xxxxxxxx > >> http://www.saout.de/mailman/listinfo/dm-crypt > > > > _______________________________________________ > dm-crypt mailing list > dm-crypt@xxxxxxxx > http://www.saout.de/mailman/listinfo/dm-crypt -- Arno Wagner, Dr. sc. techn., Dipl. Inform., Email: arno@xxxxxxxxxxx GnuPG: ID: CB5D9718 FP: 12D6 C03B 1B30 33BB 13CF B774 E35C 5FA1 CB5D 9718 ---- One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision. -- Bertrand Russell _______________________________________________ dm-crypt mailing list dm-crypt@xxxxxxxx http://www.saout.de/mailman/listinfo/dm-crypt
