[Qemu-devel] [Bug 1811543] [NEW] virtio-scsi gives improper discard sysfs entries

[James Harvey]

Public bug reported:

Apologies if this is just an inherent part of paravirtualization that
should be expected.

In my host, I have an LVM thin pool with chunk_size 128MB.  Within it, I
have a thin volume "tmp".  In the host:

# fdisk -l /dev/lvm/tmp
Disk /dev/lvm/tmp: 256 MiB, 268435456 bytes, 524288 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 262144 bytes / 134217728 bytes
Disklabel type: gpt
Disk identifier: BAE3154E-6E85-F642-8129-BAD7B58B2775

Device        Start    End Sectors  Size Type
/dev/lvm/tmp1  2048 524254  522207  255M Linux filesystem

$ lsblk
...
      └─lvm-tmp                      254:13   0   256M  0 lvm
        └─lvm-tmp1                   254:14   0   255M  0 part

$ cat /sys/dev/block/254:13/discard_alignment
0
$ cat /sys/dev/block/254:13/queue/discard_granularity
134217728
$ cat /sys/dev/block/254:13/queue/discard_max_bytes
17179869184
$ cat /sys/dev/block/254:13/queue/discard_max_hw_bytes
0
$ cat /sys/dev/block/254:13/queue/discard_zeroes_data
0

$ cat /sys/dev/block/254:14/discard_alignment
133169152
$ cat /sys/dev/block/254:14/queue/discard_granularity
134217728
$ cat /sys/dev/block/254:14/queue/discard_max_bytes
17179869184
$ cat /sys/dev/block/254:14/queue/discard_max_hw_bytes
0
$ cat /sys/dev/block/254:14/queue/discard_zeroes_data
0

If this is given to QEMU using virtio-scsi:

   -device virtio-scsi-pci,id=scsi1 \
   -drive 
driver=raw,node-name=hdb,file=/dev/lvm/tmp,if=none,discard=unmap,id=hd2 \
   -device scsi-hd,drive=hd2,bootindex=1 \

Then incorrect values are given:

$ lsblk
...
sdb     8:16   0   256M  0 disk
└─sdb1  8:17   0   255M  0 part /mnt

$ cat /sys/dev/block/8:16/discard_alignment
0
$ cat /sys/dev/block/8:16/queue/discard_granularity
4096
$ cat /sys/dev/block/8:16/queue/discard_max_bytes
1073741824
$ cat /sys/dev/block/8:16/queue/discard_max_hw_bytes
1073741824
$ cat /sys/dev/block/8:16/queue/discard_zeroes_data
0

$ cat /sys/dev/block/8:17/discard_alignment
133169152

And, there isn't even a /sys/dev/block/8:17/queue direcotry.

** Affects: qemu
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1811543

Title:
  virtio-scsi gives improper discard sysfs entries

Status in QEMU:
  New

Bug description:
  Apologies if this is just an inherent part of paravirtualization that
  should be expected.

  In my host, I have an LVM thin pool with chunk_size 128MB.  Within it,
  I have a thin volume "tmp".  In the host:

  # fdisk -l /dev/lvm/tmp
  Disk /dev/lvm/tmp: 256 MiB, 268435456 bytes, 524288 sectors
  Units: sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 4096 bytes
  I/O size (minimum/optimal): 262144 bytes / 134217728 bytes
  Disklabel type: gpt
  Disk identifier: BAE3154E-6E85-F642-8129-BAD7B58B2775

  Device        Start    End Sectors  Size Type
  /dev/lvm/tmp1  2048 524254  522207  255M Linux filesystem

  $ lsblk
  ...
        └─lvm-tmp                      254:13   0   256M  0 lvm
          └─lvm-tmp1                   254:14   0   255M  0 part

  $ cat /sys/dev/block/254:13/discard_alignment
  0
  $ cat /sys/dev/block/254:13/queue/discard_granularity
  134217728
  $ cat /sys/dev/block/254:13/queue/discard_max_bytes
  17179869184
  $ cat /sys/dev/block/254:13/queue/discard_max_hw_bytes
  0
  $ cat /sys/dev/block/254:13/queue/discard_zeroes_data
  0

  $ cat /sys/dev/block/254:14/discard_alignment
  133169152
  $ cat /sys/dev/block/254:14/queue/discard_granularity
  134217728
  $ cat /sys/dev/block/254:14/queue/discard_max_bytes
  17179869184
  $ cat /sys/dev/block/254:14/queue/discard_max_hw_bytes
  0
  $ cat /sys/dev/block/254:14/queue/discard_zeroes_data
  0

  If this is given to QEMU using virtio-scsi:

     -device virtio-scsi-pci,id=scsi1 \
     -drive 
driver=raw,node-name=hdb,file=/dev/lvm/tmp,if=none,discard=unmap,id=hd2 \
     -device scsi-hd,drive=hd2,bootindex=1 \

  Then incorrect values are given:

  $ lsblk
  ...
  sdb     8:16   0   256M  0 disk
  └─sdb1  8:17   0   255M  0 part /mnt

  $ cat /sys/dev/block/8:16/discard_alignment
  0
  $ cat /sys/dev/block/8:16/queue/discard_granularity
  4096
  $ cat /sys/dev/block/8:16/queue/discard_max_bytes
  1073741824
  $ cat /sys/dev/block/8:16/queue/discard_max_hw_bytes
  1073741824
  $ cat /sys/dev/block/8:16/queue/discard_zeroes_data
  0

  $ cat /sys/dev/block/8:17/discard_alignment
  133169152

  And, there isn't even a /sys/dev/block/8:17/queue direcotry.

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