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    SubjectRe: Software RAID 5 - Two reads are faster than one on a SW RAID5?

    On Fri, 20 Jul 2007, Lennart Sorensen wrote:

    > On Fri, Jul 20, 2007 at 09:58:50AM -0400, Justin Piszcz wrote:
    >> I have a multi-core Q6600 CPU on a 10-disk Raptor RAID 5 running XFS.
    >> I just pulled down the Debian Etch 4.0 DVD ISO's, one for x86 and one for
    >> x86_64, when I ran md5sum -c MD5SUMS, I see ~280-320MB/s. When I ran the
    >> second one I see upwards of what I should be seeing 500-520MB/s.
    >> NOTE:: These MD5 contain the 3 DVD ISO's for each platform, 6 total ISOs.
    >> I know md5sum is cpubound to a degree, do you think that is what is
    >> happening here? Each core can only sustain ~300MB/s and then with two of
    >> four cores working, it can exceed that amount or is there some similarity
    >> with RAID1 in linux compared to RAID5?
    >> With RAID1, if you use a single read thread, you will get 60-70MB/s read
    >> on a dual raptor raid1. If you use two(?) or three threads, it will read
    >> from both disks and you will see 120-140MB/s.
    >> Is there some commonality with software RAID1 and RAID5 in Linux in this
    >> regard?
    > Could you just run top while doing md5sum and see how much cpu md5sum is
    > using on the cpu it is on? After all if it says 100% for the process,
    > then yes it is cpu bound at 300MB/s, and if not then I guess there has
    > to be another explanation.

    679 abc 23 0 4092 512 432 R 92 0.0 0:02.77 md5sum

    $ /usr/bin/time md5sum debian-40r0-i386-DVD-1.iso
    79f5bcbb36335e14142fc3578b1de96e debian-40r0-i386-DVD-1.iso
    10.69user 3.52system 0:14.47elapsed 98%CPU (0avgtext+0avgdata 0maxresident)k
    0inputs+0outputs (1major+205minor)pagefaults 0swaps

    Looks like it.

    > It looks pretty likely, since I just tried running md5sum on a 130MB
    > file here, and it takes 0.444s of user cpu time and 0.068s of system
    > time to process, and I ran it multiple times so the file ended up cached
    > in ram so disk speed isn't a concern, which I figure means my system
    > runs md5sum at about 250MB/s. This is on a single core Athlon 64 3500+
    > (2.2GHz). So if you have a slightly faster as you do with a 2.4GHz Core
    > 2, then 300MB/s seems perfectly reasonable per core. I don't quite know
    > how md5 hashes work, but I really doubt they are something you are
    > likely to be able to make threaded.
    > --
    > Len Sorensen
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