[parted-devel] Writes being split based on non-aligned partition ending
Jens Rosenboom
j.rosenboom at x-ion.de
Fri Feb 12 10:49:51 UTC 2016
2016-02-12 7:59 GMT+01:00 Sitsofe Wheeler <sitsofe at gmail.com>:
> CC'ing Jens Axboe.
>
> On 11 February 2016 at 09:54, Jens Rosenboom <j.rosenboom at x-ion.de> wrote:
>> 2016-02-11 4:48 GMT+01:00 Sitsofe Wheeler <sitsofe at gmail.com>:
>>> Trying to cc the GNU parted and linux-block mailing lists.
>>>
>>> On 9 February 2016 at 13:02, Jens Rosenboom <j.rosenboom at x-ion.de> wrote:
>>>> While trying to reproduce some performance issues I have been seeing
>>>> with Ceph, I have come across a strange behaviour which is seemingly
>>>> affected only by the end point (and thereby the size) of a partition
>>>> being an odd number of sectors. Since all documentation about
>>>> alignment only refers to the starting point of the partition, this was
>>>> pretty surprising and I would like to know whether this is expected
>>>> behaviour or maybe a kernel issue.
>>>>
>>>> The command I am using is pretty simple:
>>>>
>>>> fio --rw=randwrite --size=1G --fdatasync=1 --bs=4k
>>>> --filename=/dev/sdb2 --runtime=10 --name=test
>>>>
>>>> The difference shows itself when the partition is created either by
>>>> sgdisk or by parted:
>>>>
>>>> sgdisk --new=2:6000M: /dev/sdb
>>>>
>>>> parted -s /dev/sdb mkpart osd-device-1-block 6291456000B 100%
>>>>
>>>> The difference in the partition table looks like this:
>>>>
>>>> < 2 6291456000B 1600320962559B 1594029506560B
>>>> osd-device-1-block
>>>> ---
>>>>> 2 6291456000B 1600321297919B 1594029841920B osd-device-1-block
>>>
>>> Looks like parted took you at your word when you asked for your
>>> partition at 100%. Just out of curiosity if you try and make the same
>>> partition interactively with parted do you get any warnings after
>>> making and after running align-check ?
>>
>> No warnings and everything fine for align-check. I found out that I
>> can get the same effect if I step the partition ending manually in
>> parted in 1s increments. The sequence of write sizes is 8, 1, 2, 1, 4,
>> 1, 2, 1, 8, ... which corresponds to the size (unit s) of the
>> resulting partion mod 8.
>
> OK. Could you add the output of
> grep . /sys/block/nvme0n1/queue/*size
$ grep . /sys/block/nvme0n1/queue/*size
/sys/block/nvme0n1/queue/hw_sector_size:512
/sys/block/nvme0n1/queue/logical_block_size:512
/sys/block/nvme0n1/queue/max_segment_size:65536
/sys/block/nvme0n1/queue/minimum_io_size:512
/sys/block/nvme0n1/queue/optimal_io_size:0
/sys/block/nvme0n1/queue/physical_block_size:512
$ grep . /sys/block/sdb/queue/*size
/sys/block/sdb/queue/hw_sector_size:512
/sys/block/sdb/queue/logical_block_size:512
/sys/block/sdb/queue/max_segment_size:65536
/sys/block/sdb/queue/minimum_io_size:512
/sys/block/sdb/queue/optimal_io_size:0
/sys/block/sdb/queue/physical_block_size:512
> sgdisk -D /dev/sdb
$ sgdisk -D /dev/nvme0n1
2048
$ sgdisk -D /dev/sdb
2048
> and could you post the information about the whole partition table.
In order to make sure that there is no effect from the other
partitions, I recreated to whole table from scratch:
$ parted /dev/nvme0n1 mklabel gpt
Warning: The existing disk label on /dev/nvme0n1 will be destroyed and
all data on this disk will be lost. Do you want to continue?
Yes/No? y
Information: You may need to update /etc/fstab.
$ parted /dev/nvme0n1 mkpart test1 0% 100%
Information: You may need to update /etc/fstab.
$ parted /dev/nvme0n1 unit s print
Model: Unknown (unknown)
Disk /dev/nvme0n1: 781422768s
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 2048s 781422591s 781420544s test1
Result with fio => 4k writes. Note that the ending sector in this case
is == -1 modulo 2048, making the resulting size a true multiple of
2048. Now retry with one sector less at the end:
$ parted /dev/nvme0n1 rm 1
$ parted /dev/nvme0n1 mkpart test1 2048s 781422590s
$ parted /dev/nvme0n1 unit s print
Model: Unknown (unknown)
Disk /dev/nvme0n1: 781422768s
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 2048s 781422590s 781420543s test1
Result with fio => 512b writes
> Does sgdisk create a similar problem ending if you use
> sgdisk --new=2:0 /dev/sdb
> ? It seems strange that the end of the disk (and thus a 100% sized
> partition) wouldn't end on a multiple of 4k...
$ parted /dev/nvme0n1 rm 1
Information: You may need to update /etc/fstab.
$ sgdisk --new=1:0 /dev/nvme0n1
The operation has completed successfully.
$ parted /dev/nvme0n1 unit s print
Model: Unknown (unknown)
Disk /dev/nvme0n1: 781422768s
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
Number Start End Size File system Name Flags
1 2048s 781422734s 781420687s
Result with fio => 512b writes. Note that the partition end here is at
(disk_size - 34s).
>>>> So this is really only the end of the partition that is different.
>>>> However, in the first case, the 4k writes all get broken up into 512b
>>>> writes somewhere in the kernel, as can be seen with btrace:
>>>>
>>>> 8,16 3 36 0.000102666 8184 A WS 12353985 + 1 <- (8,18) 65985
>>>> 8,16 3 37 0.000102739 8184 Q WS 12353985 + 1 [fio]
>>>> 8,16 3 38 0.000102875 8184 M WS 12353985 + 1 [fio]
>>>> 8,16 3 39 0.000103038 8184 A WS 12353986 + 1 <- (8,18) 65986
>>>> 8,16 3 40 0.000103109 8184 Q WS 12353986 + 1 [fio]
>>>> 8,16 3 41 0.000103196 8184 M WS 12353986 + 1 [fio]
>>>> 8,16 3 42 0.000103335 8184 A WS 12353987 + 1 <- (8,18) 65987
>>>> 8,16 3 43 0.000103403 8184 Q WS 12353987 + 1 [fio]
>>>> 8,16 3 44 0.000103489 8184 M WS 12353987 + 1 [fio]
>>>> 8,16 3 45 0.000103609 8184 A WS 12353988 + 1 <- (8,18) 65988
>>>> 8,16 3 46 0.000103678 8184 Q WS 12353988 + 1 [fio]
>>>> 8,16 3 47 0.000103767 8184 M WS 12353988 + 1 [fio]
>>>> 8,16 3 48 0.000103879 8184 A WS 12353989 + 1 <- (8,18) 65989
>>>> 8,16 3 49 0.000103947 8184 Q WS 12353989 + 1 [fio]
>>>> 8,16 3 50 0.000104035 8184 M WS 12353989 + 1 [fio]
>>>> 8,16 3 51 0.000104150 8184 A WS 12353990 + 1 <- (8,18) 65990
>>>> 8,16 3 52 0.000104219 8184 Q WS 12353990 + 1 [fio]
>>>> 8,16 3 53 0.000104307 8184 M WS 12353990 + 1 [fio]
>>>> 8,16 3 54 0.000104452 8184 A WS 12353991 + 1 <- (8,18) 65991
>>>> 8,16 3 55 0.000104520 8184 Q WS 12353991 + 1 [fio]
>>>> 8,16 3 56 0.000104609 8184 M WS 12353991 + 1 [fio]
>>>> 8,16 3 57 0.000104885 8184 I WS 12353984 + 8 [fio]
>>>>
>>>> whereas in the second case, I'm getting the expected 4k writes:
>>>>
>>>> 8,16 6 42 1266874889.659842036 8409 A WS 12340232 + 8 <-
>>>> (8,18) 52232
>>>> 8,16 6 43 1266874889.659842167 8409 Q WS 12340232 + 8 [fio]
>>>> 8,16 6 44 1266874889.659842393 8409 G WS 12340232 + 8 [fio]
>>>
>>> This is weird because --size=1G should mean that fio is "seeing" an
>>> aligned end. Does direct=1 with a sequential job of iodepth=1 show the
>>> problem too?
>>
>> IIUC fio uses the size only to find out where to write to, it opens
>> the block device and passes the resulting fd to the fdatasync call, so
>> the kernel will not know about what size fio thinks the device has. In
>> fact, the effect is the same without the size=1G option, I used it
>> just to make sure that the writes do not go anywhere near the badly
>> aligned partition ending.
>>
>> direct=1 kills the effect, i.e. all writes will be 4k size again.
>> Astonishingly though, sequential writes also are affected, i.e.
>> changing to rw=write in my sample above behaves the same as randwrite.
>
> Do you get this style of behaviour without fdatasync (or with larger
> values of fdatasync) too?
Wow, now you see me pretty surprised, I had checked before that
fdatasync=[2,4] did the same thing, but now it turns out that I am
seeing the 512b writes even without fdatasync at all on this NVMe
device.
In fact, if I run this test on an SSD and watch it with btrace, I also
see lots of 512b writes being queued, but again they get merged before
this has too much impact, a typical sample here looks like:
8,16 5 40466 26.397939811 22948 A WS 15489 + 1 <- (8,17) 13441
8,16 5 40467 26.397939888 22948 Q WS 15489 + 1 [fio]
8,16 5 40468 26.397939970 22948 M WS 15489 + 1 [fio]
8,16 5 40469 26.397940088 22948 A WS 15490 + 1 <- (8,17) 13442
8,16 5 40470 26.397940166 22948 Q WS 15490 + 1 [fio]
8,16 5 40471 26.397940247 22948 M WS 15490 + 1 [fio]
...
8,16 5 48524 26.399000710 22948 A WS 18175 + 1 <- (8,17) 16127
8,16 5 48525 26.399000788 22948 Q WS 18175 + 1 [fio]
8,16 5 48526 26.399000868 22948 M WS 18175 + 1 [fio]
8,16 5 48527 26.399002416 22948 A WS 18176 + 1 <- (8,17) 16128
8,16 5 48528 26.399002497 22948 Q WS 18176 + 1 [fio]
8,16 5 48529 26.399002845 22948 G WS 18176 + 1 [fio]
8,16 5 48530 26.399003324 22948 I WS 15488 + 168 [fio]
8,16 5 48531 26.399003405 22948 I WS 15656 + 168 [fio]
8,16 5 48532 26.399003449 22948 I WS 15824 + 168 [fio]
8,16 5 48533 26.399003494 22948 I WS 15992 + 168 [fio]
8,16 5 48534 26.399003535 22948 I WS 16160 + 168 [fio]
8,16 5 48535 26.399003577 22948 I WS 16328 + 168 [fio]
8,16 5 48536 26.399003622 22948 I WS 16496 + 168 [fio]
8,16 5 48537 26.399003662 22948 I WS 16664 + 168 [fio]
8,16 5 48538 26.399003702 22948 I WS 16832 + 168 [fio]
8,16 5 48539 26.399003742 22948 I WS 17000 + 168 [fio]
8,16 5 48540 26.399003782 22948 I WS 17168 + 168 [fio]
8,16 5 48541 26.399003822 22948 I WS 17336 + 168 [fio]
8,16 5 48542 26.399003862 22948 I WS 17504 + 168 [fio]
8,16 5 48543 26.399003902 22948 I WS 17672 + 168 [fio]
8,16 5 48544 26.399003942 22948 I WS 17840 + 168 [fio]
8,16 5 48545 26.399003987 22948 I WS 18008 + 168 [fio]
So I think we can forget about the fdatasync, seems that was only some
kind of colored fish. In fact, we also do not need to original writes
to be small, using bs=4M results in the same "+ 1" writes in btrace.
More information about the parted-devel
mailing list