Today Corrado Zoccolo wrote: > On Tue, Sep 15, 2009 at 9:30 AM, Tobias Oetiker wrote: > > Below is an excerpt from iostat while the test is in full swing: > > > > * 2.6.31 (8 cpu x86_64, 24 GB Ram) > > * scheduler = cfq > > * iostat -m -x dm-5 5 > > * running in parallel on 3 lvm logical volumes > >  on a single physical volume > > > > Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util > > --------------------------------------------------------------------------------------------------------- > > dm-5             0.00     0.00   26.60 7036.60     0.10    27.49     8.00   669.04   91.58   0.09  66.96 > > dm-5             0.00     0.00   63.80 2543.00     0.25     9.93     8.00  1383.63  539.28   0.36  94.64 > > dm-5             0.00     0.00   78.00 5084.60     0.30    19.86     8.00  1007.41  195.12   0.15  77.36 > > dm-5             0.00     0.00   44.00 5588.00     0.17    21.83     8.00   516.27   91.69   0.17  95.44 > > dm-5             0.00     0.00    0.00 6014.20     0.00    23.49     8.00  1331.42   66.25   0.13  76.48 > > dm-5             0.00     0.00   28.80 4491.40     0.11    17.54     8.00  1000.37  412.09   0.17  78.24 > > dm-5             0.00     0.00   36.60 6486.40     0.14    25.34     8.00   765.12  128.07   0.11  72.16 > > dm-5             0.00     0.00   33.40 5095.60     0.13    19.90     8.00   431.38   43.78   0.17  85.20 > > > > for comparison these are the numbers for seen when running the test > > with just the writing enabled > > > > Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util > > --------------------------------------------------------------------------------------------------------- > > dm-5             0.00     0.00    4.00 12047.40    0.02    47.06     8.00   989.81   79.55   0.07  79.20 > > dm-5             0.00     0.00    3.40 12399.00    0.01    48.43     8.00   977.13   72.15   0.05  58.08 > > dm-5             0.00     0.00    3.80 13130.00    0.01    51.29     8.00  1130.48   95.11   0.04  58.48 > > dm-5             0.00     0.00    2.40 5109.20     0.01    19.96     8.00   427.75   47.41   0.16  79.92 > > dm-5             0.00     0.00    3.20    0.00     0.01     0.00     8.00   290.33 148653.75 282.50  90.40 > > dm-5             0.00     0.00    3.40 5103.00     0.01    19.93     8.00   168.75   33.06   0.13  67.84 > > > > From the numbers, it appears that your controller (or the disks) is > lying about writes. They get cached (service time around 0.1ms) and > reported as completed. When the cache is full, or a flush is issued, > you get >200ms delay. > Reads, instead, have a more reasonable service time of around 2ms. > > Can you disable write cache, and see if the response times for writes > become reasonable? sure can ... the write-only results look like this. As you have predicted, the wMB has gone down to more realistic levels ... but also the await has gone way up. Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util --------------------------------------------------------------------------------------------------------- dm-18 0.00 0.00 0.00 2566.80 0.00 10.03 8.00 2224.74 737.62 0.39 100.00 dm-18 0.00 0.00 9.60 679.00 0.04 2.65 8.00 400.41 1029.73 1.35 92.80 dm-18 0.00 0.00 0.00 2080.80 0.00 8.13 8.00 906.58 456.45 0.48 100.00 dm-18 0.00 0.00 0.00 2349.20 0.00 9.18 8.00 1351.17 491.44 0.43 100.00 dm-18 0.00 0.00 3.80 665.60 0.01 2.60 8.00 906.72 1098.75 1.39 93.20 dm-18 0.00 0.00 0.00 1811.20 0.00 7.07 8.00 1008.23 725.34 0.55 100.00 dm-18 0.00 0.00 0.00 2632.60 0.00 10.28 8.00 1651.18 640.61 0.38 100.00 in the read/write test the write rate stais down, but the rMB/s is even worse and the await is also way up, so I guess the bad performance is not to blame on the the cache in the controller ... Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm  %util --------------------------------------------------------------------------------------------------------- dm-18 0.00 0.00 0.00 1225.80 0.00 4.79 8.00 1050.49 807.38 0.82 100.00 dm-18 0.00 0.00 0.00 1721.80 0.00 6.73 8.00 1823.67 807.20 0.58 100.00 dm-18 0.00 0.00 0.00 1128.00 0.00 4.41 8.00 617.94 832.52 0.89 100.00 dm-18 0.00 0.00 0.00 838.80 0.00 3.28 8.00 873.04 1056.37 1.19 100.00 dm-18 0.00 0.00 39.60 347.80 0.15 1.36 8.00 590.27 1880.05 2.57 99.68 dm-18 0.00 0.00 0.00 1626.00 0.00 6.35 8.00 983.85 452.72 0.62 100.00 dm-18 0.00 0.00 0.00 1117.00 0.00 4.36 8.00 1047.16 1117.78 0.90 100.00 dm-18 0.00 0.00 0.00 1319.20 0.00 5.15 8.00 840.64 573.39 0.76 100.00 cheers tobi -- Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland http://it.oetiker.ch tobi@oetiker.ch ++41 62 775 9902 / sb: -9900