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
2014-01-10T18:14:05Z

On 10 January 2014 16:32, Sergey Meirovich <rathamahata@gmail.com> wrote:
> Hi Jan,
>
> On 10 January 2014 12:48, Jan Kara <jack@suse.cz> wrote:
>> On Fri 10-01-14 12:36:22, Sergey Meirovich wrote:
>>> Hi Jan,
>>>
>>> On 10 January 2014 11:36, Jan Kara <jack@suse.cz> wrote:
>>> > On Thu 09-01-14 12:11:16, Sergey Meirovich wrote:
>>> ...
>>> >> 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?
>>> > So I was experimenting a bit. "sysbench prepare" seems to always do
>>> > synchronous IO from a single thread in the 'prepare' phase regardless of
>>> > the arguments. So there the reported throughput isn't really relevant.
>>> >
>>> > In the 'run' phase it obeys the arguments and indeed when I run fallocate
>>> > to preallocate files during 'run' phase, it significantly helps the
>>> > throughput (from 20 MB/s to 55 MB/s on my SATA drive).
>>>
>>> Sorry, Jan. Seems that I presented my findings in a previous mail in
>>> ambiguous style . I know that prepare phase of sysbench is
>>> synchronous/probably buffered (because I saw 512k chunks sent down to
>>> HBA)? IO. I played with blocktrace and have seen that myself during
>>> prepare:
>>>
>>> [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
>>> ...
>>>
>>> Leads to:
>>>
>>> [root@dca-poc-gtsxdb3 mnt]# blktrace -d /dev/sdg -o - | blkparse -i -
>>> | grep 'D W'
>>> 8,96 14 604 53.129805520 28114 D WS 1116160 + 1024 [sysbench]
>>> 8,96 14 607 53.129843345 28114 D WS 1120256 + 1024 [sysbench]
>>> 8,96 14 610 53.129873782 28114 D WS 1124352 + 1024 [sysbench]
>>> 8,96 14 613 53.129903703 28114 D WS 1128448 + 1024 [sysbench]
>>> 8,96 14 616 53.130957213 28114 D WS 1132544 + 1024 [sysbench]
>>> 8,96 14 619 53.130988835 28114 D WS 1136640 + 1024 [sysbench]
>>> 8,96 14 622 53.131018854 28114 D WS 1140736 + 1024 [sysbench]
>>> ...
>> Ah, ok. I misuderstood what you wrote then.
>>
>>> That result "13.042Mb/sec 3338.73 Requests/sec" was from run phase
>>> and before it fallocate had been made.
>>>
>>> blktrace from run phase looks very different. 4k as expected.
>>> [root@dca-poc-gtsxdb3 ~]# blktrace -d /dev/sdg -o - | blkparse -i - |
>>> grep 'D W'
>>> 8,96 5 3 0.000001874 28212 D WS 1847296 + 8 [sysbench]
>>> 8,96 5 7 0.001213728 28212 D WS 1847304 + 8 [sysbench]
>>> 8,96 5 11 0.002779304 28212 D WS 1847312 + 8 [sysbench]
>>> 8,96 5 15 0.004486445 28212 D WS 1847320 + 8 [sysbench]
>>> 8,96 5 19 0.006012133 28212 D WS 22691864 + 8 [sysbench]
>>> 8,96 5 23 0.007781553 28212 D WS 22691896 + 8 [sysbench]
>>> 8,96 5 27 0.009043404 28212 D WS 22691928 + 8 [sysbench]
>>> 8,96 5 31 0.010546829 28212 D WS 22691960 + 8 [sysbench]
>>> 8,96 5 35 0.012214468 28212 D WS 22691992 + 8 [sysbench]
>>> 8,96 5 39 0.013792616 28212 D WS 22692024 + 8 [sysbench]
>>> ...
>> Strange - I see:
>> 8,32 7 2 0.000086080 0 D WS 1869752 + 1024 [swapper]
>> 8,32 7 7 0.041126425 0 D WS 1874712 + 24 [swapper]
>> 8,32 7 6 0.041054543 0 D WS 1871792 + 416 [swapper]
>> 8,32 7 7 0.041126425 0 D WS 1874712 + 24 [swapper]
>> 8,32 6 118 0.042761949 28952 D WS 1875416 + 528 [sysbench]
>> 8,32 6 143 0.042995928 28952 D WS 1876888 + 48 [sysbench]
>> 8,32 5 352 0.045154160 28955 D WS 1876936 + 168 [sysbench]
>> 8,32 6 444 0.045527660 28952 D WS 1878296 + 992 [sysbench]
>> ...
>>
>> Not ideal but significantly better. The only idea I have: Didn't you run
>> fallocate(1) before you started the 'run' phase? Because 'run' phase
>> truncates the files before doing io to them. Can you check that during run
>> phase (after fallocate is run) the file size is constantly at 80MB?
>
> Jan, I believe your initial theory that append AIO is equal to
> Synchronous DIO is absolutely correct. I've given up sysbench and
> written simple dirty wrapper around io_submit() and run it against
> preallocated file.. XtremIO is doing online deduplication so results
> were wonderful
>
> 694.25 MB/s 177728.84 Req/sec

And with this simple patch to the wrapper
--- 4k.c.orig 2014-01-10 10:09:34.059797854 -0800
+++ 4k.c 2014-01-10 10:07:43.377184860 -0800
@@ -25,7 +25,7 @@
io_context_t ctx;
int ret;

- int flag = O_RDWR | O_DIRECT;
+ int flag = O_RDWR | O_DIRECT | O_CREAT;
int fd = open(FNAME, flag);
if (fd == -1) {
printf("open(%s, %d) - failed!\nExiting.\n"

To tirgger append DIO results are indeed much worse:

root@dca-poc-gtsxdb3 mnt]# rm -f 4k.data
[root@dca-poc-gtsxdb3 mnt]# /root/4k
io_submit() accepted 524288 IOs
io_getevents() returned 524288 events
time elapsed (sec.): 172.203561
bandwidth (MiB/s): 11.89
IOps: 3044.58
[root@dca-poc-gtsxdb3 mnt]#

>
> [root@dca-poc-gtsxdb3 mnt]# dd if=/dev/zero of=4k.data bs=4096 count=524288
> 524288+0 records in
> 524288+0 records out
> 2147483648 bytes (2.1 GB) copied, 5.75357 s, 373 MB/s
> [root@dca-poc-gtsxdb3 mnt]# ./4k
> io_submit() accepted 524288 IOs
> io_getevents() returned 524288 events
> time elapsed (sec.): 2.949932d
> bandwidth (MiB/s): 694.25
> IOps: 177728.84
> [root@dca-poc-gtsxdb3 mnt]#
>
> ...
> [root@dca-poc-gtsxdb3 ~]# vmstat 1
> procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> r b swpd free buff cache si so bi bo in cs us sy id wa st
> 0 0 0 260598160 260540 1896032 0 0 3 24 11 12
> 0 0 100 0 0
> 0 0 0 260598256 260544 1896032 0 0 0 16 1106 378
> 0 0 100 0 0
> 0 0 0 260598288 260544 1896032 0 0 0 0 1093 373
> 0 0 100 0 0
> 1 0 0 260499536 260544 1928368 0 0 0 293820 2782 1438
> 0 1 99 0 0
> 2 0 0 260484816 260544 1928804 0 0 0 820152 5575 3146
> 0 3 97 0 0
> 1 0 0 260481680 260544 1928804 0 0 0 710028 4844 2947
> 0 3 97 0 0
> 1 0 0 260548384 260544 1928804 0 0 0 273168 2187 1744
> 0 2 98 0 0
> 0 0 0 260549088 260544 1928804 0 0 0 4 1156 426
> 0 0 100 0 0
> 0 0 0 260549472 260544 1928804 0 0 0 0 1082 328
> 0 0 100 0 0
> ^C
> [root@dca-poc-gtsxdb3 ~]#
>
>
> ========================== io_submit() wrapper =============================
> #define _GNU_SOURCE
>
> #include <errno.h>
> #include <libaio.h>
>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
> #include <unistd.h>
> #include <sys/time.h>
>
>
> #define FNAME "4k.data"
> #define IOSIZE 4096
> #define REQUESTS 524288
>
> /* gcc 4k.c -std=gnu99 -laio -o 4k */
>
> int main(void) {
> io_context_t ctx;
> int ret;
>
> int flag = O_RDWR | O_DIRECT;
> int fd = open(FNAME, flag);
> if (fd == -1) {
> printf("open(%s, %d) - failed!\nExiting.\n"
> "If file doesn't exist please precreate it "
> "with dd if=/dev/zero of=%s bs=%d count=%d\n",
> FNAME, flag, FNAME, IOSIZE, REQUESTS);
> return errno;
> }
>
> memset(&ctx, 0, sizeof(io_context_t));
> if (io_setup(REQUESTS, &ctx)) {
> printf("io_setup(%d, &ctx) failed\n", REQUESTS);
> return -ret;
> }
>
> void *mem = NULL;
> posix_memalign(&mem, 4096, IOSIZE);
> memset(mem, 9, IOSIZE);
> struct iocb *aio = malloc(sizeof(struct iocb) * REQUESTS);
> memset(aio, 0, sizeof(struct iocb) * REQUESTS);
> struct iocb **lio = malloc(sizeof(void *) * REQUESTS);
> memset(lio, 0, sizeof(void *) * REQUESTS);
> struct io_event *event = malloc(sizeof(struct io_event) * REQUESTS);
> memset(event, 0, sizeof(struct io_event) * REQUESTS);
>
> for (int i = 0; i < REQUESTS; i++) {
> io_prep_pwrite(&aio[i], fd, mem, IOSIZE, i * IOSIZE);
> lio[i] = &aio[i];
> }
> struct timeval start, end;
> gettimeofday(&start, NULL);
> ret = io_submit(ctx, REQUESTS, lio);
> printf("io_submit() accepted %d IOs\n", ret);
> fdatasync(fd);
>
> ret = io_getevents(ctx, REQUESTS, REQUESTS, event, NULL);
> printf("io_getevents() returned %d events\n", ret);
> gettimeofday(&end, NULL);
>
> double elapsed = (end.tv_sec - start.tv_sec) +
> ((end.tv_usec - start.tv_usec)/1000000.0);
> printf("time elapsed (sec.):\t%2f\n", elapsed);
> printf("bandwidth (MiB/s):\t%.2f\n",
> (double) (((long long) IOSIZE * REQUESTS) / (1024 * 1024))
> / elapsed);
> printf("IOps:\t\t\t%.2f\n", (double) REQUESTS
> / elapsed);
>
> if (io_destroy(ctx)) {
> perror("io_destroy");
> return -1;
> }
>
> free(aio);
> free(lio);
> free(event);
>
> return 0;
> }
>
>
>>
>> Honza
>> --
>> Jan Kara <jack@suse.cz>
>> SUSE Labs, CR