Re: [Qemu-devel] Bug#757927: [qemu-kvm] TRIM (discard=unmap) broken in 2.1

[Roman Mamedov]

On Wed, 13 Aug 2014 10:05:51 +0400
Michael Tokarev <address@hidden> wrote:

> BTW, I found a system here with an SSD which supports discard, and tried my
> guests against it -- everything works fine when I use sata/ahci, but it breaks
> indeed when using the default IDE interface.

Hello,

Thanks for working on this! Sorry for not being able to test an updated
version sooner.

Something to note, you do not need any TRIM-capable hardware such as SSD on the
host to take advantage of TRIM passthrough. Just place the RAW format VM disk
image on any filesystem which supports sparse files (e.g. Ext4 or Btrfs), and
when the guest system issues a TRIM, when using "discard=unmap" the VM image
file will be "re-sparsifyed" on the host filesystem, deallocating all the
TRIMed areas from disk storage, in effect it will take much less space on disk
than before:
http://s.lowendshare.com/7/1407910491.666.2014-07-16T130602Z-trim.png

I was only successful in using this with the virtual IDE interface disks on
Qemu/KVM 2.0, the virtio mode does not seem to support TRIM; did not try with
sata/ahci.

-- 
With respect,
Roman

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