Re: [Qemu-devel] extremely low IOPS performance of QCOW2 image format on an SSD RAID1

[Kevin Wolf <kwolf@redhat.com>]

Am 24.06.2014 um 09:25 hat lihuiba geschrieben:
> >Can you confirm that making L2_CACHE_SIZE much bigger solves the
> >problem?
> Yes, it is confirmed.
> When I run fio randread with a 7GB or 8GB size, result is close to that of raw image.
> But when the size is increased to 9GB, the result drops dramatically. And I have modified
> qcow2-cache.c to print a log when cache misses. When testing the 9GB size, there are
> lots of cache misses shown.
> 
> >You also have the optional of specifying the cluster size when creating
> >the qcow2 image file.  A larger cluster size reduces the amount of
> >metadata overhead and therefore increases cache hits.
> I didn't find any command-line option to increase the size of the cache, so I increased
> cluster_size to 1MB or 2MB. This worked very well for me.
> 
> BTW
> qemu-img of version 1.7.1 has bug when creating qcow2 image with options preallocation=metadata
> and cluster_size > 64K. It reports:
> 
> 
> # qemu-img create -f qcow2 -o preallocation=metadata,cluster_size=1M asdf.qcow2 100G
> Formatting 'asdf.qcow2', fmt=qcow2 size=107374182400 encryption=off cluster_size=1048576 preallocation='metadata' lazy_refcounts=off 
> qemu-img: block/qcow2-cluster.c:1196: qcow2_alloc_cluster_offset: Assertion `n_start * (1ULL << 9) == offset_into_cluster(s, offset)' failed.

This appears to be fixed in current git master.

Kevin