Hi, 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? Arno 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 -- 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
