Frantisek Rysanek
Oh I see. You're saying that
-drive ...,discard=ignore
Does not prevent Windows (guest OS) from trying the
discard=trim=unmap on the virtio disk device, merely those operations
get ignored by QEMU.
Thus, Windows thinks it's trimming, while in reality it's just
wasting time on void request+response transactions...
You're saying that apparently, virtio-blk or virtio-scsi still
advertise the availability of that command (via SCSI INQUIRY / Vital
Product Data / LBPU bit), or Windows understand that this is a virtio
drive and just assumes that discard is available...
Yes that is what I mean. qemu
advertises the discard feature to upper vm. the driver follows the advertisements(unless you use a very old driver) . so both windows and linux think they can trim.
I think windows hurts more since linux didn't do disk optimization automatically by default.
Hmm. I've just tried googling, if there's a way to do SCSI Command
pass-through from the guest OS to the host-side physical SCSI device
(should also work for SATA, if I understand correctly).
Here is what I've found:
https://gist.github.com/amshinde/b9b2763cb3f6752508ca522f41b583ca
i.e. it's three cmdline args to qemu-system: 1x -drive + 2x -device.
Personally I've never done this.
I always just specify the device node for a "file" in "raw" mode.
Also, I'd expect that the SCSI command passthrough could actually be
slower for routine operation, because it doesn't make use of the
block-level cache integrated with the host's (HV's) memory
management...
I still think the SCSI or IDE style emulated controller might just do
the trick, but I have no clue if your RedHat build of QEMU still
offers those options.
In Debian, I tend to compile my own from upstream source code,
because the distro package tends to be old and lack recent features.
Fortunately my machines using qemu are on the experimental side :-)
and can take a bit of punk / no harm done if something doesn't work.
Thanks for the info. I will try it when I have time. but in reality I won't use sata or ide for the vm.
I run qemu directly for checking/debugging the vm situation. but for daily usage I still use libvirt to manage vm.