Il 28-06-2018 12:44 Daniel P. Berrangé ha scritto:
There is always a performance distinction between raw and qcow2, but it
is
much less these days with qcow2v3 than it was with the original qcow2
design.
Sure, but especially with random reads/writes over large LBA range the
difference is noticeable [1]. Moreover, if something goes wrong, a RAW
file can be inspected with standard block device tools. As a reference
point, both oVirt and RHEL uses RAW files for base disk images.
It's not only performance related, but it regards thin-provision also.
Why the wizard should automatically select fat provisioning based on
image format? What if I want thin-provisioning using filesystem's sparse
file support via RAW files?
This is really tangential. virt-manager chose to use internal snapshots
because they were easy to support, but it could equally use external
snapshots. This shouldn't have a bearing on other choices - if the
internal snapshotting is unacceptable due to the guest pause, this
needs addressing regardless of allocation.
I agree, but currently the wizard force you to do a choice between:
a) sparse Qcow2 file, with (sometime dangerous?) internal snapshot
support;
b) fully allocated RAW files, with *no* external snapshot support.
As you can see, it is virt-manager itself that entangles the choices
regarding file format/allocation/snapshot support.
And external snapshot support in virt-manager would be *super* cool ;)
Using qcow2 doesn't require you to use cow at the disk image layer - it
simply gives you the ability, should you want to. So you don't get
double
cow by default
I badly expressed the idea, sorry. Writing to a *snapshotted* Qcow2 file
causes double CoW; on the other hand, writing to an un-snapshotted Qcow2
file only causes double block allocation.
Which widely used modern filesystems still don't support fallocate. It
is
essentially a standard feature on any modern production quality
filesystem
these days.
True, with an exception: ZFS. And it is a *big* exception. Moreover, why
allocate all data by default when using RAW files? What about thin
images?
What really strikes me is that the checkbox *was* here in previous
virt-manager releases. Did it caused confusion or some other problems?
Thanks.
[1] https://www.linux-kvm.org/images/9/92/Qcow2-why-not.pdf
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