Hyper-V uses a VHD or VHDX file on the host as the underlying storage for a
virtual disk.  The VHD/VHDX file format is a sparse format where real disk
space on the host is assigned in chunks that the VHD/VHDX file format calls
the BlockSize.  This BlockSize is not to be confused with the 512-byte (or
4096-byte) sector size of the underlying storage device.  The default block
size for a new VHD/VHDX file is 32 Mbytes.  When a guest VM touches any
disk space within a 32 Mbyte chunk of the VHD/VHDX file, Hyper-V allocates
32 Mbytes of real disk space for that section of the VHD/VHDX. Similarly,
if a discard operation is done that covers an entire 32 Mbyte chunk,
Hyper-V will free the real disk space for that portion of the VHD/VHDX.
This BlockSize is surfaced in Linux as the "discard_granularity" in
/sys/block/sd<x>/queue, which makes sense.

Hyper-V also has differencing disks that can overlay a VHD/VHDX file to
capture changes to the VHD/VHDX while preserving the original VHD/VHDX.
One example of this differencing functionality is for VM snapshots.  When a
snapshot is created, a differencing disk is created.  If the snapshot is
rolled back, Hyper-V can just delete the differencing disk, and the VM will
see the original disk contents at the time the snapshot was taken.
Differencing disks are used in other scenarios as well.

The BlockSize for a differencing disk defaults to 2 Mbytes, not 32 Mbytes.
The smaller default is used because changes to differencing disks are
typically scattered all over, and Hyper-V doesn't want to allocate 32
Mbytes of real disk space for a stray write here or there.  The smaller
BlockSize provides more efficient use of real disk space.

When a differencing disk is added to a VHD/VHDX, Hyper-V reports
UNIT_ATTENTION with a sense code indicating "Operating parameters have
changed", because the value of discard_granularity should be changed to 2
Mbytes. When the differencing disk is removed, discard_granularity should
be changed back to 32 Mbytes.  However, current code simply reports a
message from scsi_report_sense() and the value of
/sys/block/sd<x>/queue/discard_granularity is not updated. The message
isn't very actionable by a sysadmin.

Fix this by having the storvsc driver check for the sense code indicating
that the underly VHD/VHDX block size has changed, and do a rescan of the
device to pick up the new discard_granularity.  With this change the entire
transition to/from differencing disks is handled automatically and
transparently, with no confusing messages being output.

Link: https://lore.kernel.org/r/1677516514-86060-1-git-send-email-mikelley@microsoft.comSigned-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>