Re: Terrible performance of sequential O_DIRECT 4k writes in SAN environment. ~3 times slower then Solars 10 with the same HBA/Storage.

Sergey Meirovich
2014-01-09T10:11:37Z

Hi Jan,
On 8 January 2014 22:55, Jan Kara <jack@suse.cz> wrote:
>
>> So far I've seen so massive degradation only in SAN environment. I
>> started my investigation with RHEL6.5 kernel so below table is from it
>> but the trend is the same as for mainline it seems.
>>
>> Chunk size Bandwidth MiB/s
>> ================================
>> 64M 512
>> 32M 510
>> 16M 492
>> 8M 451
>> 4M 436
>> 2M 350
>> 1M 256
>> 512K 191
>> 256K 165
>> 128K 142
>> 64K 101
>> 32K 65
>> 16K 39
>> 8K 20
>> 4K 11
> Yes, that's expected. The latency to complete a request consists of some
> fixed overhead + time to write data. So for small request sizes the latency
> is constant (corresponding to bandwidth growing linearly with the request
> size) and for larger request sizes latency somewhat grows so bandwidth grows
> slower and slower (as the time to write the data forms larger and larger
> part of the total latency)...

Why these latencies are not hurting random 4k on XtremIO so much? It
gave 451.11Mb/sec 115485.02 Requests/sec

I've done preallocation on fnic/XtremIO as Christoph suggested.

[root@dca-poc-gtsxdb3 mnt]# sysbench --max-requests=0
--file-extra-flags=direct --test=fileio --num-threads=4
--file-total-size=10G --file-io-mode=async --file-async-backlog=1024
--file-rw-ratio=1 --file-fsync-freq=0 --max-requests=0
--file-test-mode=seqwr --max-time=100 --file-block-size=4K prepare
sysbench 0.4.12: multi-threaded system evaluation benchmark

128 files, 81920Kb each, 10240Mb total
Creating files for the test...
[root@dca-poc-gtsxdb3 mnt]# du -k test_file.* | awk '{print $1}' |sort |uniq
81920
[root@dca-poc-gtsxdb3 mnt]# fallocate -l 81920k test_file.*

Results: 13.042Mb/sec 3338.73 Requests/sec

Probably sysbench is still triggering append DIO scenario. Will say
simple wrapper over io_submit() against already preallocated (and even
filled with data) file provide much better throughput if your theory
is valid?

======================== iostat -x -t out ================================
01/09/2014 02:00:29 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.02 0.00 1.22 0.20 0.00 98.57

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.10 168.30 0.80 1346.40
8.00 0.07 0.39 0.39 6.56
sdg 0.00 0.00 0.10 168.20 0.80 1345.60
8.00 0.06 0.34 0.34 5.66
sdo 0.00 0.00 0.10 168.30 0.80 1346.40
8.00 0.07 0.41 0.41 6.97
sdp 0.00 0.00 0.10 168.20 0.80 1345.60
8.00 0.07 0.41 0.41 6.88

01/09/2014 02:00:39 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.08 0.00 1.03 1.69 0.00 97.20

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.00 903.90 0.00 7231.20
8.00 0.36 0.40 0.39 35.69
sdg 0.00 0.00 0.00 903.90 0.00 7231.20
8.00 0.29 0.33 0.33 29.38
sdo 0.00 0.00 0.00 903.80 0.00 7230.40
8.00 0.31 0.34 0.34 30.98
sdp 0.00 0.00 0.00 903.90 0.00 7231.20
8.00 0.36 0.40 0.40 36.14

01/09/2014 02:00:49 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.05 0.00 0.89 1.62 0.00 97.44

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.10 962.00 0.80 7696.00
8.00 0.36 0.37 0.37 35.85
sdg 0.00 0.00 0.10 962.00 0.80 7696.00
8.00 0.31 0.32 0.32 30.57
sdo 0.00 0.00 0.10 962.10 0.80 7696.80
8.00 0.35 0.36 0.36 34.56
sdp 0.00 0.00 0.10 962.10 0.80 7696.80
8.00 0.38 0.40 0.40 38.39

01/09/2014 02:00:59 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.05 0.00 0.93 1.99 0.00 97.02

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.00 914.00 0.00 7312.00
8.00 0.34 0.37 0.37 33.78
sdg 0.00 0.00 0.00 914.10 0.00 7312.80
8.00 0.30 0.33 0.33 29.92
sdo 0.00 0.00 0.00 914.00 0.00 7312.00
8.00 0.31 0.34 0.34 31.00
sdp 0.00 0.00 0.00 914.00 0.00 7312.00
8.00 0.37 0.40 0.40 36.65

01/09/2014 02:01:09 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.07 0.00 0.99 1.58 0.00 97.35

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.10 982.70 0.80 7861.60
8.00 0.36 0.37 0.37 36.36
sdg 0.00 0.00 0.10 982.60 0.80 7860.80
8.00 0.32 0.33 0.33 32.21
sdo 0.00 0.00 0.10 982.70 0.80 7861.60
8.00 0.33 0.34 0.34 33.01
sdp 0.00 0.00 0.10 982.70 0.80 7861.60
8.00 0.40 0.41 0.40 39.76

01/09/2014 02:01:19 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.04 0.00 0.80 2.01 0.00 97.15

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.00 767.60 0.00 6140.80
8.00 0.33 0.43 0.43 32.75
sdg 0.00 0.00 0.00 767.60 0.00 6140.80
8.00 0.30 0.39 0.39 29.57
sdo 0.00 0.00 0.00 767.60 0.00 6140.80
8.00 0.30 0.39 0.38 29.48
sdp 0.00 0.00 0.00 767.60 0.00 6140.80
8.00 0.34 0.45 0.45 34.37

01/09/2014 02:01:29 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.07 0.00 0.76 2.12 0.00 97.05

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.10 762.00 0.80 6096.00
8.00 0.32 0.42 0.42 31.86
sdg 0.00 0.00 0.10 762.00 0.80 6096.00
8.00 0.30 0.39 0.39 29.86
sdo 0.00 0.00 0.10 761.90 0.80 6095.20
8.00 0.32 0.42 0.42 32.07
sdp 0.00 0.00 0.10 761.90 0.80 6095.20
8.00 0.34 0.44 0.44 33.59

01/09/2014 02:01:39 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.05 0.00 0.82 1.79 0.00 97.34

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.00 779.40 0.00 6235.20
8.00 0.35 0.45 0.45 34.87
sdg 0.00 0.00 0.00 779.40 0.00 6235.20
8.00 0.32 0.41 0.40 31.53
sdo 0.00 0.00 0.00 779.50 0.00 6236.00
8.00 0.32 0.41 0.41 32.11
sdp 0.00 0.00 0.00 779.50 0.00 6236.00
8.00 0.37 0.47 0.47 36.43

01/09/2014 02:01:49 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.06 0.00 0.94 1.57 0.00 97.44

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.10 837.80 0.80 6702.40
8.00 0.33 0.40 0.40 33.25
sdg 0.00 0.00 0.10 837.80 0.80 6702.40
8.00 0.28 0.34 0.34 28.46
sdo 0.00 0.00 0.00 837.80 0.00 6702.40
8.00 0.32 0.38 0.38 31.77
sdp 0.00 0.00 0.00 837.80 0.00 6702.40
8.00 0.34 0.41 0.41 34.25

01/09/2014 02:01:59 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.05 0.00 0.97 1.82 0.00 97.16

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.00 871.80 0.00 7081.40
8.12 0.34 0.38 0.38 33.21
sdg 0.00 0.00 0.00 871.80 0.00 7090.00
8.13 0.31 0.35 0.35 30.30
sdo 0.00 0.00 0.10 871.80 0.80 7128.40
8.18 0.32 0.37 0.37 31.86
sdp 0.00 0.00 0.10 871.80 0.80 7129.40
8.18 0.36 0.41 0.41 35.78

01/09/2014 02:02:09 AM
avg-cpu: %user %nice %system %iowait %steal %idle
0.03 0.00 0.46 0.83 0.00 98.69

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s
avgrq-sz avgqu-sz await svctm %util
sdh 0.00 0.00 0.00 400.40 0.00 3203.20
8.00 0.17 0.42 0.42 16.80
sdg 0.00 0.00 0.00 400.40 0.00 3203.20
8.00 0.14 0.36 0.36 14.25
sdo 0.00 0.00 0.00 400.40 0.00 3203.20
8.00 0.15 0.37 0.37 14.67
sdp 0.00 0.00 0.00 400.40 0.00 3203.20
8.00 0.16 0.40 0.40 16.05