Re: [Qemu-discuss] qcow2 performance improve

[yang.bin18]

> > > > When use cluster_size=1M qcow2 newly created to test "32k rand
> > > > write" in the VM,We get low performance within VM。But there
> > > > are huge “write” requests in HOST which falls into qemu
> > > > stack of"perform_cow".
> > > The reason why this happens is because clusters are the smallest
> > > units of allocation in qcow2. So if you write a new cluster you
> > > have to fill it completely with data. If there was any existing
> > > data (backing file, snapshot), you need to copy it from there.
> > > If cluster_size=1M then every time you send a write request that
> > > needs to allocate a new cluster then you need to write 1MB. It
> > > doesn't matter if the write request is 32KB, 4KB or 512 bytes.

> > But Our image has not backing file or snapshot.

>If there is no backing file or snapshot you still need to fill the
>cluster with zeroes, and that's going to be slower with larger
>clusters.

    If not fill zeroes and only write guest data ,what`s wrong could happen ?


>More information here:

>   https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg01229.html

>There's work to improve that scenario (allocating a new cluster with no
>backing file or snapshots), here are some patches:

>   http://lists.nongnu.org/archive/html/qemu-block/2018-05/msg00180.html

>More information about those patches here:

>   http://lists.nongnu.org/archive/html/qemu-devel/2017-11/msg00059.html

>And that's for the "no backing file" case. The more general solution
>would be the subcluster allocation feature that I mentioned in my
>previous e-mail:

>   https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg01033.html

>I gave a presentation last year at the KVM Forum about performance
>with the qcow2 format:

>   https://www.youtube.com/watch?v=kmUxIOTiGNo

>Another earlier talk on the same topic by Max Reitz and Kevin Wolf:

>   https://www.youtube.com/watch?v=svMpxzl9yI4

>I hope that helps,

>Berto