Exadata: disk level statistics

This is the fourth post on a serie of postings on how to get measurements out of the cell server, which is the storage layer of the Oracle Exadata database machine. Up until now, I have looked at the measurement of the kind of IOs Exadata receives, the latencies of the IOs as as done by the cell server, and the mechanism Exadata uses to overcome overloaded CPUs on the cell layer.

This post is about the statistics on the disk devices on the operating system, which the cell server also collects and uses. The disk statistics are ideal to combine with the IO latency statistics.

This is how a dump of the collected statistics (which is called “devio_stats”) is invoked on the cell server, using cellcli:

alter cell events="immediate cellsrv.cellsrv_dump('devio_stats',0)"; 

This will output the name of the thread-log file, in which the “devio_stats” dump has been made.

This is a quick peek at the statistics this dump provides (first 10 lines):

[IOSTAT] Dump IO device stats for the last 1800 seconds
2013-10-28 04:57:39.679590*: Dump sequence #34:
[IOSTAT] Device - /dev/sda
ServiceTime Latency AverageRQ numReads numWrites DMWG numDmwgPeers numDmwgPeersFl trigerConfine avgSrvcTimeDmwg avgSrvcTimeDmwgFl
0.000000 0.000000 10 0 6 0 0 0 0 0.000000 0.000000
0.111111 0.111111 15 7 38 0 0 0 0 0.000000 0.000000
0.000000 0.000000 8 4 8 0 0 0 0 0.000000 0.000000
0.000000 0.000000 31 0 23 0 0 0 0 0.000000 0.000000
0.000000 0.000000 8 0 1 0 0 0 0 0.000000 0.000000
0.058824 0.058824 25 0 17 0 0 0 0 0.000000 0.000000
etc.

These are the devices for which the cell server keeps statistics:

grep \/dev\/ /opt/oracle/cell11.2.3.2.1_LINUX.X64_130109/log/diag/asm/cell/enkcel01/trace/svtrc_15737_85.trc
[IOSTAT] Device - /dev/sda
[IOSTAT] Device - /dev/sda3
[IOSTAT] Device - /dev/sdb
[IOSTAT] Device - /dev/sdb3
[IOSTAT] Device - /dev/sdc
[IOSTAT] Device - /dev/sde
[IOSTAT] Device - /dev/sdd
[IOSTAT] Device - /dev/sdf
[IOSTAT] Device - /dev/sdg
[IOSTAT] Device - /dev/sdh
[IOSTAT] Device - /dev/sdi
[IOSTAT] Device - /dev/sdj
[IOSTAT] Device - /dev/sdk
[IOSTAT] Device - /dev/sdl
[IOSTAT] Device - /dev/sdm
[IOSTAT] Device - /dev/sdn
[IOSTAT] Device - /dev/sdo
[IOSTAT] Device - /dev/sdp
[IOSTAT] Device - /dev/sdq
[IOSTAT] Device - /dev/sdr
[IOSTAT] Device - /dev/sds
[IOSTAT] Device - /dev/sdt
[IOSTAT] Device - /dev/sdu

What is of interest here is that if the cell disk is allocated inside a partition instead of the whole disk, the cell server will keep statistics on both the entire device (/dev/sda, dev/sdb) and the partition (/dev/sda3, dev/sdb3). Also, the statistics are kept on both the rotating disks and the flash disks, as you would expect.

When looking in the “devio_stats” dump, there are a few other things which are worthy to notice. The lines with statistics do not have timestamp or other time indicator, it’s only statistics. The lines are displayed per device, with the newest line on top. The dump indicates it dumps the IO device statistics which the cell keeps for the last 1800 seconds (30 minutes). If you count the number of lines which (apparently) are kept by the cell server, the count is 599, not 1800. If you divide the time by the number of samples, it appears the cell takes a device statistics snapshot every 3 seconds. The cell server picks up the disk statistics from /proc/diskstats. Also, mind the cell measures the differences between two periods in time, which means the numbers are averages over a period of 3 seconds.

Two other things are listed in the statistics: ‘trigerConfine’ (which probably should be “triggerConfine”), which is a mechanism for Oracle to manage under performing disks.
The other thing is “DMWG”. At this moment I am aware DMWG means “Disk Media Working Group”, and works with the concept of peers.

To get a better understanding of what the difference is between the ServiceTime and Latency columns, see this excellent writeup on IO statistics from Bart Sjerps. You can exchange the ServiceTime for svctm of iostat or storage wait as Bart calls it, and Latency for await or host wait as Bart calls it.

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Exadata: measuring IO latencies experienced by the cell server

Exadata is about doing IO. I think if there’s one thing people know about Exadata, that’s it. Exadata brings (part of the) processing potentially closer to the storage media, which will be rotating disks for most (Exadata) users, and optionally can be flash.

But with Exadata, you either do normal alias regular IO, which will probably be single block IO, or multiblock IO, which hopefully gets offloaded. The single block reads are hopefully coming from the flashcache, which can be known by looking at v$sysstat/v$sesstat at the statistic (“cell flash cache read hits”), not directly by looking at the IO related views. To understand the composition of the response time of a smartscan, there is even lesser instrumentation in the database (for background, look at this blogpost, where is shown that the smartscan wait does not detail any of the steps done in a smartscan. In other words: if you experience performance differences on Exadata, and the waits point towards IO, there’s not much analysis which can be done to dig deeper.

Luckily, the Exadata storage server provides a very helpful dump which details IO latencies of what the cell considers celldisks (which are both flash and rotating disks). The dump provides:

– IO size by number of reads and writes
– IO size versus latency for reads and writes
– IO size versus pending IO count for reads and writes
– IO size versus pending IO sizes for reads and writes

This is how this dump is executed (in the cellcli of course):

alter cell events="immediate cellsrv.cellsrv_dump('iolstats',0)";

As with the other dumps, the cellcli provides the name of the trace file where the requested dump has been written to. If we look inside this trace file, this is how an IO latencies dump looks like:

IO length (bytes):          Num read IOs:       Num write IOs:
[    512 -    1023)                212184               104402
[   1024 -    2047)                     0               138812
[   2048 -    4095)                     0               166282
[   4096 -    8191)                    35               134095
[   8192 -   16383)                498831               466674
[  16384 -   32767)                  2006                73433
[  32768 -   65535)                    91                15072
[  65536 -  131071)                   303                 4769
[ 131072 -  262143)                   297                 6376
[ 262144 -  524287)                  1160                  230
[ 524288 - 1048575)                  2278                   36
[1048576 - 2097151)                   459                   21

Average IO-latency distribution stats for CDisk CD_02_enkcel01

Number of Reads iosize-latency distribution
IO len(B)\IO lat(us) || [       32 | [       64 | [      128 | [      256 | [      512 | [     1024 | [     2048 | [     4096 | [     8192 | [    16384 | [    32768 | [    65536 | [   131072 | [   262144 | [   524288 |
                     ||        63) |       127) |       255) |       511) |      1023) |      2047) |      4095) |      8191) |     16383) |     32767) |     65535) |    131071) |    262143) |    524287) |   1048575) |
---------------------||------------|------------|------------|------------|------------|------------|------------|------------|------------|------------|------------|------------|------------|------------|------------|
[     512,     1023) ||      31075 |      14592 |      69575 |      55370 |       7744 |        385 |        725 |       6489 |       7044 |      11663 |       4030 |       1770 |       1310 |        408 |          4 |
[    4096,     8191) ||          0 |          6 |          5 |          6 |          0 |          0 |          0 |          0 |          7 |          8 |          3 |          0 |          0 |          0 |          0 |
[    8192,    16383) ||         66 |        101 |       3189 |       6347 |        717 |       1826 |      23168 |     124246 |     191169 |      79157 |      37032 |      18508 |      12778 |        526 |          1 |
[   16384,    32767) ||         22 |         46 |         22 |       1403 |         90 |         46 |         57 |         65 |         77 |        124 |         39 |          5 |          7 |          3 |          0 |
...

What struck me as odd, is the name of the celldisk (CD_02_enkcel01 here) is below the first table (IO lengths) about this celldisk(!)

In my previous post we saw a command to reset statistics (a cell events command). There is a command to reset the statistics for this specific dump (‘iolstats’) too (to be executed in the cellcli of course):

alter cell events = "immediate cellsrv.cellsrv_resetstats(iolstats)";

Next, I executed a smartscan

IO length (bytes):          Num read IOs:       Num write IOs:
[   4096 -    8191)                     0                   24
[ 524288 - 1048575)                     8                    0
[1048576 - 2097151)                   208                    0

Average IO-latency distribution stats for CDisk CD_02_enkcel01

Number of Reads iosize-latency distribution
IO len(B)\IO lat(us) || [     4096 | [     8192 | [    16384 | [    32768 | [    65536 | [   131072 | [   262144 |
                     ||      8191) |     16383) |     32767) |     65535) |    131071) |    262143) |    524287) |
---------------------||------------|------------|------------|------------|------------|------------|------------|
[  524288,  1048575) ||          0 |          0 |          3 |          1 |          0 |          2 |          2 |
[ 1048576,  2097151) ||          1 |          3 |         15 |         22 |         89 |         59 |         19 |

As can be seen, the statistics have been reset (on the local cell/storage server!). This makes diagnosing the physical IO subsystem of Exadata possible!