Re: Large qcow2 performance

[Alberto Garcia]

On Mon, Sep 23, 2019 at 03:17:39PM -0500, Andrew Martin wrote:

> I am considering creating a VM with a large qcow2, around 6TB in
> size. This Linux VM will be reading and writing a lot of small
> files to this very large qcow2 using the VirtIO driver. Are there
> any concerns or guidance about creating such a large qcow2, or
> performance issues with small files on a 6TB qcow2?

Briefly:

there's two important parameters that can affect performance:
cluster_size and l2_cache_size.

cluster_size is set when you create the image, l2_cache_size is a
runtime option.

If the L2 cache size is not large enough, I/O performance will suffer.
Check here for a summary (the defaults have changed since then, but
the basic description of the problem is correct):

   
https://blogs.igalia.com/berto/2015/12/17/improving-disk-io-performance-in-qemu-2-5-with-the-qcow2-l2-cache/

With small clusters you need a larger L2 cache, that is: QEMU needs
more memory in order to get the same performance.

Images with large clusters use less memory, but there are two
drawbacks:

- The cluster is the minimum unit of allocation, so even if you write
  512 bytes QEMU needs to allocate one complete cluster and fill it
  with data. This makes *allocation* much slower the larger the
  cluster size.

  See the table here for an example, but focus only on the first and
  last columns and ignore the two in the middle:

     https://lists.gnu.org/archive/html/qemu-block/2019-07/msg00496.html

  After a cluster is allocated an image with large clusters should
  give you the same (or possibly better) performance.

- Because of what I just said, images with large clusters waste more
  space on disk.

A hybrid approach that combines the benefits of large clusters
regarding memory usage with the benefits of small clusters regarding
allocation performance is currently under development, and I plan to
send patches soon.

Berto