[Lustre-discuss] HW RAID - fragmented I/O
Kevin Van Maren
kevin.van.maren at oracle.com
Wed Jun 8 09:30:08 PDT 2011
Yep, with 1.8.5 the problem is most likely in the (mpt2sas) driver, not
in the rest of the kernel. Driver limits are not normally noticed by
(non-Lustre) people, because the default kernel limits IO to 512KB.
May want to see Bug 22850 for the changes required eg, for the
Emulex/lpfc driver.
Glancing at the stock RHEL5 kernel, it looks like the issue is
MPT2SAS_SG_DEPTH, which is limited to 128. This appears to be set to
match the default kernel limit, but it is possible there is also a
driver/HW limit. You should be able to increase that to 256 and see if
it works...
Also note that the size buckets are power-of-2, so a "1MB" entry is any
IO > 512KB and <= 1MB.
If you can't get the driver to reliably do full 1MB IOs, change to a
64KB chunk and set max_sectors_kb to 512. This will help ensure you get
aligned, full-stripe writes.
Kevin
Wojciech Turek wrote:
> I am setting up a new lustre filesystem using LSI engenio based disk
> enclosures with integrated dual RAID controllers. I configured disks
> into 8+2 RAID6 groups using 128kb segment size (chunk size). This
> hardware uses mpt2sas kernel module on the Linux host side. I use the
> whole block device for an OST (to avoid any alignment issues). When
> running sgpdd-survey I can see high through numbers (~3GB/s write,
> 5GB/s read), Also controllers stats show that number of IOPS = number
> of MB/s. However as soon as I put ldiskfs on the OSTs, obdfilter shows
> slower results (~2GB/s write , ~2GB/s read ) and controller stats show
> more then double IOPS than MB/s. Looking at output from iostat -m -x 1
> and brw_stats I can see that a large number of I/O operations are
> smaller than 1MB, mostly 512kb. I know that there was some work done
> on optimising the kernel block device layer to process 1MB I/O
> requests and that those changes were committed to Lustre 1.8.5. Thus I
> guess this I/O chopping happens below the Lustre stack, maybe in the
> mpt2sas driver?
>
> I am hoping that someone in Lustre community can shed some light on to
> my problem.
>
> In my setup I use:
> Lustre 1.8.5
> CentOS-5.5
>
> Some parameters I tuned from defaults in CentOS:
> deadline I/O scheduler
>
> max_hw_sectors_kb=4096
> max_sectors_kb=1024
>
>
> brw_stats output
> --
>
> find /proc/fs/lustre/obdfilter/ -name "testfs-OST*" | while read ost;
> do cat $ost/brw_stats ; done | grep "disk I/O size" -A9
>
> disk I/O size ios % cum % | ios % cum %
> 4K: 206 0 0 | 521 0 0
> 8K: 224 0 0 | 595 0 1
> 16K: 105 0 1 | 479 0 1
> 32K: 140 0 1 | 1108 1 3
> 64K: 231 0 1 | 1470 1 4
> 128K: 536 1 2 | 2259 2 7
> 256K: 1762 3 6 | 5644 6 14
> 512K: 31574 64 71 | 30431 35 50
> 1M: 14200 28 100 | 42143 49 100
> --
> disk I/O size ios % cum % | ios % cum %
> 4K: 187 0 0 | 457 0 0
> 8K: 244 0 0 | 598 0 1
> 16K: 109 0 1 | 481 0 1
> 32K: 129 0 1 | 1100 1 3
> 64K: 222 0 1 | 1408 1 4
> 128K: 514 1 2 | 2291 2 7
> 256K: 1718 3 6 | 5652 6 14
> 512K: 32222 65 72 | 29810 35 49
> 1M: 13654 27 100 | 42202 50 100
> --
> disk I/O size ios % cum % | ios % cum %
> 4K: 196 0 0 | 551 0 0
> 8K: 206 0 0 | 551 0 1
> 16K: 79 0 0 | 513 0 1
> 32K: 136 0 1 | 1048 1 3
> 64K: 232 0 1 | 1278 1 4
> 128K: 540 1 2 | 2172 2 7
> 256K: 1681 3 6 | 5679 6 13
> 512K: 31842 64 71 | 31705 37 51
> 1M: 14077 28 100 | 41789 48 100
> --
> disk I/O size ios % cum % | ios % cum %
> 4K: 190 0 0 | 486 0 0
> 8K: 200 0 0 | 547 0 1
> 16K: 93 0 0 | 448 0 1
> 32K: 141 0 1 | 1029 1 3
> 64K: 240 0 1 | 1283 1 4
> 128K: 558 1 2 | 2125 2 7
> 256K: 1716 3 6 | 5400 6 13
> 512K: 31476 64 70 | 29029 35 48
> 1M: 14366 29 100 | 42454 51 100
> --
> disk I/O size ios % cum % | ios % cum %
> 4K: 209 0 0 | 511 0 0
> 8K: 195 0 0 | 621 0 1
> 16K: 79 0 0 | 558 0 1
> 32K: 134 0 1 | 1135 1 3
> 64K: 245 0 1 | 1390 1 4
> 128K: 509 1 2 | 2219 2 7
> 256K: 1715 3 6 | 5687 6 14
> 512K: 31784 64 71 | 31172 36 50
> 1M: 14112 28 100 | 41719 49 100
> --
> disk I/O size ios % cum % | ios % cum %
> 4K: 201 0 0 | 500 0 0
> 8K: 241 0 0 | 604 0 1
> 16K: 82 0 1 | 584 0 1
> 32K: 130 0 1 | 1092 1 3
> 64K: 230 0 1 | 1331 1 4
> 128K: 547 1 2 | 2253 2 7
> 256K: 1695 3 6 | 5634 6 14
> 512K: 31501 64 70 | 31836 37 51
> 1M: 14343 29 100 | 41517 48 100
>
>
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