Re: [Qemu-discuss] Virtio-blk and trim

[Brian Rak]

It does make a lot of sense, if you're using LVM as a backend for the 
guest.  LVM handles discard commands by zeroing out the relevant data.  
This means that if you take a backup of the guest from the host, you 
aren't backing up all the old, unused data.

Running something like fstrim from the host will not do this.

On 10/1/2014 4:24 PM, Tony Su wrote:
>   I don't know that it makes any sense to run a TRIM/discard function
> in a virtual file system.
> It only makes sense for the command to be applied directly to the disk
> at the hardware I/O level.
>
> So, if you run the command on your HostOS regularly, it should addres
> the needs for all GuestOS as well as the HostOS.
>
> Tony
>
> On Wed, Oct 1, 2014 at 1:19 PM, George Shuklin <address@hidden> wrote:
> > It's better to name it 'discard', because TRIM a very specific ATA command.
> > It done completely different in SCSI and slightly different in SD cards. But
> > linux call it 'discard', or, to be precise, BLKDISCARD (name of ioctl).
> >
> >
> > On 10/01/2014 02:21 AM, Brian Rak wrote:
> > > Are there any plans to bring trim support to Virtio-blk?  We've got a
> > > couple things that would be made nicer by having working trim support.
> > >
> > > I see that it's implemented in virtio-scsi, but there's a decent
> > > performance hit for switching over to that.