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Tomasu 05-30-2009 01:03 AM

extremly slow random read on new raid5 array
 
I just setup a new md raid5 array with 4 Seagate 7200.12 1TB disks. The sequential read and write is pretty amazing to me, but the random read is going very slowly. iostat says its going at about 500KB/s vs the 300MB/s+ and 170MB/s+ for sequential read and write respectively.

I've seen people report they get random reads in the double digit MB range, and I would have at least liked to see 50MB/s or something along those lines.

The raid array is setup with a 64KB chunk, I'm using ext4 (the mkfs and fsck times for the full array with ext3 would have killed me, and I'm starting to dislike the occasional corruption and slowness I get with XFS).

What am I missing? There's got to be something I misconfigured some place.

Tomasu 05-30-2009 02:47 PM

My iozone test finished, so I'll post the results:

Code:

natasha:/mnt/test-data# time iozone -a -s 8G
        Iozone: Performance Test of File I/O
                Version $Revision: 3.308 $ 
                Compiled for 64 bit mode. 
                Build: linux               
 
        Contributors:William Norcott, Don Capps, Isom Crawford, Kirby Collins
                    Al Slater, Scott Rhine, Mike Wisner, Ken Goss
                    Steve Landherr, Brad Smith, Mark Kelly, Dr. Alain CYR,
                    Randy Dunlap, Mark Montague, Dan Million, Gavin Brebner,
                    Jean-Marc Zucconi, Jeff Blomberg, Benny Halevy,
                    Erik Habbinga, Kris Strecker, Walter Wong, Joshua Root.
 
        Run began: Fri May 29 21:16:37 2009
 
        Auto Mode
        File size set to 8388608 KB
        Command line used: iozone -a -s 8G
        Output is in Kbytes/sec
        Time Resolution = 0.000001 seconds.
        Processor cache size set to 1024 Kbytes.
        Processor cache line size set to 32 bytes.
        File stride size set to 17 * record size.
                                                            random  random    bkwd  record  stride
              KB  reclen  write rewrite    read    reread    read  write    read  rewrite    read  fwrite frewrite  fread  freread
        8388608      4  174685  213251  355843  365144    586    1284  12223  2280563    16758  218247  263474  358397  366568
        8388608      8  223099  240634  355358  365280    1168    2397  18998  2642433    18052  210898  233876  350771  365105
        8388608      16  226324  229919  365406  360961    2315    4390  26815  2810571    8341  218916  251165  361090  357974
        8388608      32  218344  255547  359488  365617    4532    7680  28945  3173752    9059  211130  247218  351760  366038
        8388608      64  214133  216581  360049  357493    8666  12576  35039  3067238    11112  215186  258125  352486  364836
        8388608    128  215983  263689  361029  355805  14172  22884  44144  3134393    17952  220013  262199  359932  358492
        8388608    256  219437  221301  361070  365407  24231  27027  55767  3249251    31228  219924  250765  355612  368312
        8388608    512  224317  249989  362780  359418  44359  44989  70847  3114559    75262  218529  256738  352011  359514
        8388608    1024  220018  245323  363428  357883  77843  71840  102576  2949858    89856  222584  246536  357558  358643
        8388608    2048  223986  257198  362738  364082  127198  100024  143284  2867324  140889  217681  248418  353124  364953
        8388608    4096  223963  240834  360078  360076  197791  121379  203363  2333949  211259  218647  259376  352413  358489
        8388608    8192  217639  235952  356011  356751  246708  143259  262451  2173888  264425  214914  234224  356112  367021
        8388608  16384  217533  231808  352947  367823  301731  161226  296074  2091368  297689  221477  243934  364667  365455
 
iozone test complete.
 
real    967m19.809s
user    1m28.250s
sys    54m3.239s

Out of the tests, only about the last 4 are adequate. I'd really like to see some better times from the smaller record sizes as well.


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