Re: [Bug Report] Discard bios cannot be correctly merged in blk-mq

Hi all

Thanks for reporting about this. I did a test in my environment.
time blkdiscard /dev/nvme5n1  (477GB)
real    0m0.398s
time blkdiscard /dev/md0
real    9m16.569s

I'm not familiar with the block layer codes. I'll try to understand
the codes related with discard request and
try to fix this problem.

I have a question for raid5 discard, it needs to consider more than
raid0 and raid10. For example, there is a raid5 with 3 disks.
D11 D21 P1 (stripe size is 4KB)
D12 D22 P2
D13 D23 P3
D14 D24 P4
...  (chunk size is 512KB)
If there is a discard request on D13 and D14, and there is no discard
request on D23 D24. It can't send
discard request to D13 and D14, right? P3 = D23 xor D13. If we discard
D13 and disk2 is broken, it can't
get the right data from D13 and P3. The discard request on D13 can
write 0 to the discard region, right?

If so, it can handle a discard bio at a time that is big enough at
least to contain the data. (data disks * chunk size). In this case the
size is 1024KB (512KB*2).

Regards
Xiao


On Wed, Jun 9, 2021 at 10:40 AM Wang Shanker [off-list ref] wrote:

quoted

2021年06月09日 08:41，Ming Lei [off-list ref] 写道：

On Tue, Jun 08, 2021 at 11:49:04PM +0800, Wang Shanker wrote:

quoted

Actually, what are received by the nvme controller are discard requests
with 128 segments of 4k, instead of one segment of 512k.

Right, I am just wondering if this way makes a difference wrt. single
range/segment discard request from device viewpoint, but anyway it is
better to send less segment.

It would be meaningful if more than queue_max_discard_segments() bio's
are sent and merged into big segments.

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Similarly, the problem with scsi devices can be emulated using the following
options for qemu:

      -device virtio-scsi,id=scsi \
      -device scsi-hd,drive=nvme1,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME1 \
      -device scsi-hd,drive=nvme2,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME2 \
      -device scsi-hd,drive=nvme3,bus=scsi.0,logical_block_size=4096,discard_granularity=2097152,physical_block_size=4096,serial=NVME3 \
      -trace scsi_disk_emulate_command_UNMAP,file=scsitrace.log


Despite the discovery, I cannot come up with a proper fix of this issue due
to my lack of familiarity of the block subsystem. I expect your kind feedback
on this. Thanks in advance.

In the above setting and raid456 test, I observe that rq->nr_phys_segments can
reach 128, but queue_max_discard_segments() reports 256. So discard
request size can be 512KB, which is the max size when you run 1MB discard on
raid456. However, if the discard length on raid456 is increased, the
current way will become inefficient.

Exactly.

I suggest that bio's can be merged and be calculated as one segment if they are
contiguous and contain no data.

Fine.

quoted

And I also discovered later that, even normal long write requests, e.g.
a 10m write, will be split into 4k bio's. The maximum number of bio's which can
be merged into one request is limited by queue_max_segments, regardless
of whether those bio's are contiguous. In my test environment, for scsi devices,
queue_max_segments can be 254, which means about 1m size of requests. For nvme
devices(e.g. Intel DC P4610), queue_max_segments is only 33 since their mdts is 5,
which results in only 132k of requests.

Here what matters is queue_max_discard_segments().

Here I was considering normal write/read bio's, since I first took it for granted
that normal write/read IOs would be optimal in raid456, and finally discovered
that those 4k IOs can only be merged into not-so-big requests.

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So, I would also suggest that raid456 should be improved to issue bigger bio's to
underlying drives.

Right, that should be root solution.

Cc Xiao, I remembered that he worked on this area.

Many thanks for looking into this issue.

Cheers,

Miao Wang