qcow2 perfomance: read-only IO on the guest generates high write IO on the host

[Christopher Pereira]

Hi,

I'm reading a directory with 5.000.000 files (2,4 GB) inside a guest using "find | grep -c".

On the host I saw high write IO (40 MB/s !) during over 1 hour using virt-top.

I later repeated the read-only operation inside the guest and no additional data was written on the host. The operation took only some seconds.

I believe QEMU was creating some kind of cache or metadata map the first time I accessed the inodes.

But I wonder why the cache or metadata map wasn't available the first time and why QEMU had to recreate it?

The VM has "compressed base <- snap 1" and base was converted without prealloc.

Is it because we created the base using convert without metadata prealloc and so the metadata map got lost?

I will do some experiments soon using convert + metadata prealloc and probably find out myself, but I will happy to read your comments and gain some additional insights.
If it the problem persists, I would try again without compression.

Additional info:

• Guest fs is xfs.
  
• (I believe the snapshot didn't significantly increase in size, but I would need to double check)
• This is a production host with old QEMU emulator version 2.3.0 (qemu-kvm-ev-2.3.0-31.el7_2.10.1)