Re: [libvirt] [PATCH 2/3] virStorageFileResize: fallocate the whole capacity
On 09/25/2017 11:46 AM, Ján Tomko wrote:
We have been trying to implement the ALLOCATE flag to mean
Is this the colloquial "we"? ;-)
"the volume should be fully allocated after the resize".
commit id 'aa2a4cff' added support for ALLOCATE, but it seems that code
was wrong, then?
The docs are somewhat vague regarding the ALLOCATE flag:
virStorageVolResize:
...
* Normally, the operation will attempt to affect capacity with a minimum
* impact on allocation (that is, the default operation favors a sparse
* resize). If @flags contains VIR_STORAGE_VOL_RESIZE_ALLOCATE, then the
* operation will ensure that allocation is sufficient for the new
* capacity; this may make the operation take noticeably longer.
and:
VIR_STORAGE_VOL_RESIZE_ALLOCATE = 1 << 0, /* force allocation of new size */
What really isn't clear is whether the new allocation wants to preserve
some sort of sparseness factor as well. IOW: If you have a sparse file
before you start, then shouldn't you have a sparse file when you're done?
Since commit b0579ed9 we do not allocate from the existing
capacity, but from the existing allocation value.
However this value is a total of all the allocated bytes,
not an offset.
For a sparsely allocated file:
$ perl -e 'print "x"x8192;' > vol1
$ fallocate -p -o 0 -l 4096 vol1
Treating allocation as an offset would result in an incompletely
allocated file:
$ virsh vol-resize vol1 --pool default 16384 --allocate
Capacity: 16.00 KiB
Allocation: 12.00 KiB
Yep, would have expected an allocation of 16KiB here...
Call fallocate from zero on the whole requested capacity to fully
allocate the file.
FWIW: complete output/example because I'm a visual learner...
Before:
$ perl -e 'print "x"x8192;' > /path/to/default/vol1
$ virsh pool-refresh default
$ virsh vol-info default vol1
Capacity: 8.00 KiB
Allocation: 8.00 KiB
$ fallocate -p -o 0 -l 4096 /path/to/default/vol1
$ virsh vol-info vol1 default
Capacity: 8.00 KiB
Allocation: 4.00 KiB
$ virsh vol-resize vol1 --pool default 16384 --allocate
$ virsh vol-info vol1 default
Capacity: 16.00 KiB
Allocation: 12.00 KiB
After the patch:
Only the vol-resize output changes to:
Capacity: 16.00 KiB
Allocation: 16.00 KiB
Which would seem to be correct. Still I wonder if the initial changes
got things "wrong" by trying to be cute and adding even though DELTA is
dealt with much earlier.
The question being - should the original code adjusted both capacity and
allocation getting the difference between the two as the additional
capacity in order to preserve whatever sparseness may have existed
previously or in this case a capacity=24KiB and Allocation=20KiB. Oh
well we may never know...
---
src/storage/storage_util.c | 3 +--
src/util/virstoragefile.c | 8 +-------
src/util/virstoragefile.h | 1 -
3 files changed, 2 insertions(+), 10 deletions(-)
I guess it's a long way of me giving:
Reviewed-by: John Ferlan <jferlan(a)redhat.com>
But I needed to convince myself first... and if you feel the need to
alter the commit message a bit to pull in more details that's even
better (for me), but I suppose the current one works too.
John