[Lustre-discuss] Fragmented I/O

[Kevin Van Maren]

You didn't say, but I think they are LSI-based: are you using the mptsas 
driver with the PERC cards?  Which driver version?

First, max_sectors_kb should normally be set to a power of 2 number, 
like 256, over an odd size like 320.  This number should also match the 
native raid size of the device, to avoid read-modify-write cycles.  (See 
Bug 22886 on why not to make it > 1024 in general).

See Bug 17086 for patches to increase the max_sectors_kb limitation for 
the mptsas driver to 1MB, or the true hardware maximum, rather than a 
driver limit; however, the hardware may still be limited to sizes < 1MB.

Also, to clarify the sizes: the smallest bucket >= transfer_size is the 
one incremented, so a 320KB IO increments the 512KB bucket.  Since your 
HW says it can only do a 320KB IO, there will never be a 1MB IO.

You may want to instrument your HBA driver to see what is going on (ie, 
why the max_hw_sectors_kb is < 1024).

Kevin


Kevin Hildebrand wrote:
> Hi, I'm having some performance issues on my Lustre filesystem and it 
> looks to me like it's related to I/Os getting fragmented before being 
> written to disk, but I can't figure out why.  This system is RHEL5, 
> running Lustre 1.8.4.
>
> All of my OSTs look pretty much the same-
>
>                             read      |     write
> pages per bulk r/w     rpcs  % cum % |  rpcs  % cum %
> 1:                   88811  38  38   | 46375  17  17
> 2:                    1497   0  38   | 7733   2  20
> 4:                    1161   0  39   | 1840   0  21
> 8:                    1168   0  39   | 7148   2  24
> 16:                    922   0  40   | 3297   1  25
> 32:                    979   0  40   | 7602   2  28
> 64:                   1576   0  41   | 9046   3  31
> 128:                  7063   3  44   | 16284   6  37
> 256:                129282  55 100   | 162090  62 100
>
>
>                             read      |     write
> disk fragmented I/Os   ios   % cum % |  ios   % cum %
> 0:                   51181  22  22   |    0   0   0
> 1:                   45280  19  42   | 82206  31  31
> 2:                   16615   7  49   | 29108  11  42
> 3:                    3425   1  50   | 17392   6  49
> 4:                  110445  48  98   | 129481  49  98
> 5:                    1661   0  99   | 2702   1  99
>
>                             read      |     write
> disk I/O size          ios   % cum % |  ios   % cum %
> 4K:                  45889   8   8   | 56240   7   7
> 8K:                   3658   0   8   | 6416   0   8
> 16K:                  7956   1  10   | 4703   0   9
> 32K:                  4527   0  11   | 11951   1  10
> 64K:                114369  20  31   | 134128  18  29
> 128K:                 5095   0  32   | 17229   2  31
> 256K:                 7164   1  33   | 30826   4  35
> 512K:               369512  66 100   | 465719  64 100
>
> Oddly, there's no 1024K row in the I/O size table...
>
>
> ...and these seem small to me as well, but I can't seem to change them. 
> Writing new values to either doesn't change anything.
>
> # cat /sys/block/sdb/queue/max_hw_sectors_kb
> 320
> # cat /sys/block/sdb/queue/max_sectors_kb
> 320
>
> Hardware in question is DELL PERC 6/E and DELL PERC H800 RAID 
> controllers, with MD1000 and MD1200 arrays, respectively.
>
>
> Any clues on where I should look next?
>
> Thanks,
>
> Kevin
>
> Kevin Hildebrand
> University of Maryland, College Park
> Office of Information Technology
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