Re: [Qemu-devel] SSD Trim Support- Current State?

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/03/2013 21:58, d hee ha scritto:
> What is the current state of SSD Trim/Discard support  for a Qemu/KVM
> guests? 
> 
> I run a Qemi/ KVM host wtih these kvm guests...they are all Linux os's
> and use raw images: 1) 20 gig Linux web-server 2) Subsonic music
> streaming server for my Android phone 3) Postfix server for automated
> emails 4) Linux guest for sandbox for testing. All of this is on my on
> personal system as a hobby and not a a business "production" system. If
> I run them off a ssd drive without the trim support, will I see
> performance impacted as the guest blocks slowly fill up without being
> marked for reuse?

DISCARD is finally supported in qemu.git.  It is experimental in all
released versions.  To get trim support you need to do all of the following:

2) use virtio-scsi or another SCSI HBA (IDE would work too, but it is slow);

2) compile qemu.git from master and add ",discard=on" to the "-drive"
option; or for released versions add ",discard_granularity=4096" to the
"-device" options for disks.  For more informations on "-drive" and
"-device" check out the docs/qdev-device-use.txt file.

3) mount the filesystems with the discard option in the guest, or
periodically run fstrim in the guest;

4) either run the images directly from a partition or logical volume, or
mount the filesystems with the discard option in the host, or
periodically run fstrim in the host.

Paolo