Il giorno mer 28 nov 2018 alle ore 15:10 Daniel P. Berrangé <berrange@xxxxxxxxxx> ha scritto: > > On Wed, Nov 28, 2018 at 03:05:29PM +0100, Vincenzo Romano wrote: > > Il giorno mer 28 nov 2018 alle ore 14:41 Eric Blake > > > > I doesn't make any real difference. It just puzzles me. > > > > > > The difference is whether you choose to preallocate an image. There are > > > performance tradeoffs for doing so (a preallocated image is faster than > > > a sparse one when it comes to writing to previously unused portions of > > > the disk, but occupies more host space as a result - also, a > > > preallocated image tends to have less fragmentation and less chance of > > > hitting ENOSPC). > > > > Yes, but I've never seen (so far) a sparse QCOW file. After all, QCOW > > was introduced also to let users > > use file systems not supporting "sparse files". > > qcow2 files can be sparse if you request it to preallocate metadata > only: > > No preallocation -> small file size, small allocation size: > > $ qemu-img create -f qcow2 demo.qcow2 1G > Formatting 'demo.qcow2', fmt=qcow2 size=1073741824 cluster_size=65536 lazy_refcounts=off refcount_bits=16 > $ ls -alhs demo.qcow2 > 196K -rw-r--r--. 1 berrange berrange 193K Nov 28 14:07 demo.qcow2 > > Metadata prealloc -> full file size, small allocation size (ie sparse) > > $ qemu-img create -f qcow2 -o preallocation=metadata demo.qcow2 1G > Formatting 'demo.qcow2', fmt=qcow2 size=1073741824 cluster_size=65536 preallocation=metadata lazy_refcounts=off refcount_bits=16 > $ ls -alhs demo.qcow2 > 328K -rw-r--r--. 1 berrange berrange 1.1G Nov 28 14:07 demo.qcow2 > > Full preallocate -> full file size, full allocation size: > > $ qemu-img create -f qcow2 -o preallocation=full demo.qcow2 1G > Formatting 'demo.qcow2', fmt=qcow2 size=1073741824 cluster_size=65536 preallocation=full lazy_refcounts=off refcount_bits=16 > $ ls -alhs demo.qcow2 > 1.1G -rw-r--r--. 1 berrange berrange 1.1G Nov 28 14:08 demo.qcow2 > > > qcow2 files also become sparse if you have support for discard and the > guest OS trims data blocks. I think the documentation (https://qemu.weilnetz.de/doc/qemu-doc.html#qemu_005fimg_005finvocation) needs some improvement. Great explanation, though: thanks a lot! -- Vincenzo Romano - NotOrAnd.IT Information Technologies -- NON QVIETIS MARIBVS NAVTA PERITVS _______________________________________________ virt-tools-list mailing list virt-tools-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/virt-tools-list
