diff options
Diffstat (limited to 'results/scraper/launchpad-without-comments/1538541')
| -rw-r--r-- | results/scraper/launchpad-without-comments/1538541 | 25 |
1 files changed, 25 insertions, 0 deletions
diff --git a/results/scraper/launchpad-without-comments/1538541 b/results/scraper/launchpad-without-comments/1538541 new file mode 100644 index 000000000..667b5f417 --- /dev/null +++ b/results/scraper/launchpad-without-comments/1538541 @@ -0,0 +1,25 @@ +qcow2 rejects request to use preallocation with backing file + +The 'preallocation=full' option to qemu-img / qcow2 block driver instructs QEMU to fully allocate the host file to the maximum size needed by the logical disk size. + +$ qemu-img create -f qcow2 -o preallocation=full base.qcow2 200M +Formatting 'base.qcow2', fmt=qcow2 size=209715200 encryption=off cluster_size=65536 preallocation='full' lazy_refcounts=off refcount_bits=16 + +$ ls -alhs base.qcow2 +201M -rw-r--r--. 1 berrange berrange 201M Jan 27 12:49 base.qcow2 + + +When specifying a backing file for the qcow2 file, however, it rejects the preallocation request + +$ qemu-img create -f qcow2 -o preallocation=full,backing_file=base.qcow2 front.qcow2 200M +Formatting 'front.qcow2', fmt=qcow2 size=209715200 backing_file='base.qcow2' encryption=off cluster_size=65536 preallocation='full' lazy_refcounts=off refcount_bits=16 +qemu-img: front.qcow2: Backing file and preallocation cannot be used at the same time + + +It might seem like requesting full preallocation is redundant because most data associated with the image will be present in the backing file, as so the top layer is unlikely to ever need the full preallocation. Rejecting this, however, means it is not (officially) possible to reserve disk space for the top layer to guarantee that future copy-on-writes will never get ENOSPC. + +OpenStack in particular uses backing files with all images, in order to avoid the I/O overhead of copying the backing file contents into the per-VM disk image. It, however, still wants to have a guarantee that the per-VM image will never hit an ENOSPC scenario. + +Currently it has to hack around QEMU's refusal to allow backing_file + preallocation, by calling 'fallocate' on the qcow2 file after it has been created. This is an inexact fix though, because it doesn't take account of fact that qcow2 metadata can takes some MBs of space. + +Thus, it would like to see preallocation=full supported in combination with backing files. \ No newline at end of file |