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

From: Christopher Pereira <kripper@imatronix.cl>
To: qemu-devel@nongnu.org
Subject: qcow2 perfomance: read-only IO on the guest generates high write IO on the host
Date: Wed, 11 Aug 2021 07:36:33 -0400	[thread overview]
Message-ID: <55980bc8-97ad-77a4-1bb7-a086f2712ea1@imatronix.cl> (raw)

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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)

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