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

[Brian Rak]

So, I'm saying LVM on the host.  If you have trim/discard enabled, when 
the guest trims a block, LVM marks that space as not used on the host.

It works like this:

* Take a guest running on a LVM LV (so, LVM on the host) with a 50GB 
disk.  Assume 1GB in use
* Write 30 GB to that disk, and delete it
* With trim enabled, you'll end up with the LV using 1GB of space
* Without trim, the LV would still be using 31GB of space.

I think that if you enable the 'issue_discards' option for LVM, it'll 
even pass those commands down to the physical storage.

So, discard would be very, very useful for guests.


On 10/3/2014 1:38 PM, Tony Su wrote:
>   So Brian, just clarifying...
>
> You really are talking about LVM as configured in the Guest and not
> LVM configured in the Host (and may or may not be exposed to the Guest
> as raw, bare metal storage)?
>
> If any commands which are executed on a virtual fs actually are passed
> to the physical fs, that's something new to me.
>
> Tony