[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