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)
