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.
