Public bug reported:
The nova/virt/libvirt/imagebackend.py file contains code for pre-
allocating image files, so that sufficient space is guaranteed available
for the VM's full disk size. As well as preventing accidental out-of-
space conditions, this is important for maximising performance.
It currently just calls 'qemu-img info' to get the virtual image size,
and then calls 'fallocate' to allocate file system blocks to match this
size. This is fine for raw files, but is significantly flawed for qcow2
files.
- QCow2 files have extra space overhead beyond the logical image size.
There is header metadata, cluster lookup tables and so forth. As a
result Nova is not allocating sufficient space for qcow2 files.
- The qcow2 internal metadata is not being pre-allocated. Since the
allocation was done as the FS level, qcow2 file doesn't know it has all
this space pre-allocated, so its internal metadata still setup to assume
it is a (mostly) empty file. This will have a runtime performance
overhead on guest writes, since further internal metadata will have to
be updated.
- The file will suffer from fragmentation. Again since the internal
metadat does not know the FS has full pre-allocated its size, as the
guest writes blocks, they will be allocated in the internal metadata on
a first-come-first-served basis. This will caused the layout of the
virtual disk blocks to be random inside the qcow2 file. This causes
significant fragmentation which will be bad for performance.
The solution to all these problems is to use qemu-img to do the
preallocation, so the qcow2 file internals are fully in sync with the
filesystem size.
** Affects: nova
Importance: Undecided
Status: New
** Tags: libvirt
** Tags added: libvirt
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https://bugs.launchpad.net/bugs/1510328
Title:
Nova pre-allocation of qcow2 is flawed
Status in OpenStack Compute (nova):
New
Bug description:
The nova/virt/libvirt/imagebackend.py file contains code for pre-
allocating image files, so that sufficient space is guaranteed
available for the VM's full disk size. As well as preventing
accidental out-of-space conditions, this is important for maximising
performance.
It currently just calls 'qemu-img info' to get the virtual image size,
and then calls 'fallocate' to allocate file system blocks to match
this size. This is fine for raw files, but is significantly flawed for
qcow2 files.
- QCow2 files have extra space overhead beyond the logical image
size. There is header metadata, cluster lookup tables and so forth. As
a result Nova is not allocating sufficient space for qcow2 files.
- The qcow2 internal metadata is not being pre-allocated. Since the
allocation was done as the FS level, qcow2 file doesn't know it has
all this space pre-allocated, so its internal metadata still setup to
assume it is a (mostly) empty file. This will have a runtime
performance overhead on guest writes, since further internal metadata
will have to be updated.
- The file will suffer from fragmentation. Again since the internal
metadat does not know the FS has full pre-allocated its size, as the
guest writes blocks, they will be allocated in the internal metadata
on a first-come-first-served basis. This will caused the layout of the
virtual disk blocks to be random inside the qcow2 file. This causes
significant fragmentation which will be bad for performance.
The solution to all these problems is to use qemu-img to do the
preallocation, so the qcow2 file internals are fully in sync with the
filesystem size.
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