[Lustre-discuss] Fragmented I/O

Kevin Van Maren kevin.van.maren at oracle.com
Thu May 12 06:16:31 PDT 2011


Kevin Hildebrand wrote:
>
> The PERC 6 and H800 use megaraid_sas, I'm currently running 
> 00.00.04.17-RH1.
>
> The max_sectors numbers (320) are what is being set by default- I am 
> able to set it to something smaller than 320, but not larger.

Right.  You can not set max_sectors_kb larger than max_hw_sectors_kb 
(Linux normally defaults most drivers to 512, but Lustre sets them to be 
the same): you may want to instrument your HBA driver to see what is 
going on (ie, why the max_hw_sectors_kb is < 1024).  I don't know if it 
is due to a driver limitation or a true hardware limit.

Most drivers have a limit of 512KB by default; see Bug 22850 for the 
patches that fixed the QLogic and Emulex fibre channel drivers.

Kevin

> Kevin
>
> On Wed, 11 May 2011, Kevin Van Maren wrote:
>
>> 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
>>> _______________________________________________
>>> Lustre-discuss mailing list
>>> Lustre-discuss at lists.lustre.org
>>> http://lists.lustre.org/mailman/listinfo/lustre-discuss
>>>
>>
>>




More information about the lustre-discuss mailing list