Re: [Qemu-devel] TRIM/DISCARD/UNMAP support on qemu-nbd

["Richard W.M. Jones" <rjones@redhat.com>]

Using qcow2 format, it also doesn't appear to work:

$ /tmp/sparsetest.sh 
Formatting '/tmp/test1', fmt=qcow2 size=104857600 encryption=off cluster_size=65536 lazy_refcounts=off 
Formatting '/tmp/test2', fmt=qcow2 size=104857600 encryption=off cluster_size=65536 lazy_refcounts=off 
136K	   /tmp/test1
136K	   /tmp/test2
Read Capacity results:
   Protection: prot_en=0, p_type=0, p_i_exponent=0
   Logical block provisioning: lbpme=1, lbprz=0
   Last logical block address=204799 (0x31fff), Number of logical blocks=204800
   Logical block length=512 bytes
   Logical blocks per physical block exponent=0
   Lowest aligned logical block address=0
Hence:
   Device size: 104857600 bytes, 100.0 MiB, 0.10 GB
Block limits VPD page (SBC):
  Write same no zero (WSNZ): 1
  Maximum compare and write length: 0 blocks
  Optimal transfer length granularity: 0 blocks
  Maximum transfer length: 0 blocks
  Optimal transfer length: 0 blocks
  Maximum prefetch length: 0 blocks
  Maximum unmap LBA count: 2097152
  Maximum unmap block descriptor count: 255
  Optimal unmap granularity: 8
  Unmap granularity alignment valid: 0
  Unmap granularity alignment: 0
  Maximum write same length: 0x0 blocks

17M	  /tmp/test1
17M	  /tmp/test2

$ ll -h /tmp/test{1,2}
-rw-r--r--. 1 rjones rjones 17M Jan  7 21:24 /tmp/test1
-rw-r--r--. 1 rjones rjones 17M Jan  7 21:24 /tmp/test2
$ qemu-img info /tmp/test1
image: /tmp/test1
file format: qcow2
virtual size: 100M (104857600 bytes)
disk size: 16M
cluster_size: 65536
$ qemu-img info /tmp/test2
image: /tmp/test2
file format: qcow2
virtual size: 100M (104857600 bytes)
disk size: 16M
cluster_size: 65536

----------------------------------------------------------------------

A frustrating aspect of this is there's no diagnostics or way to probe
if UNMAP is supported all the way through.

This will be critical for virt-sparsify, since we'd like to be able to
tell the user in advance whether or not in-place sparsification is
going to work, and even better, why not.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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