Re: [Qemu-discuss] Discard disk image holes for sparse shrink

[Narcis Garcia]

Thank you.

* I use RAW images because of it's easier to offline mount/chroot when
some issue happens.
* I call directly qemu instead of libvirt

I'll try to parse these parameters to a qemu-system call.


El 8/4/19 a les 22:03, Friedrich Oslage ha escrit:
> Yes.
> 
> You'll have to use the virtio-scsi driver, to my knowledge it's the only
> driver that supports block discards.
> 
> Quick example:
> 
> # create a new empty disk
> $ qemu-img create -f qcow2 disk.qcow2 10G
> 
> # check size, should only be a few kilobytes
> $ du -sh disk.qcow2
> 
> # add it to your vm
> $ virtsh edit your-vm
>   <disk type='block' device='disk'>
>     <driver name='qemu' type='qcow2' discard='unmap'
> detect_zeroes='unmap'/>
>     <source dev='/tmp/disk.qcow2'/>
>     <target dev='sda' bus='scsi'/>
>     <address type='drive' controller='0' bus='0' target='0' unit='0'/>
>   </disk>
>   <controller type='scsi' index='0' model='virtio-scsi'>
>     <driver queues='4'/>
>   </controller>
> 
> # in your vm, format and mount it
> $ mkfs.ext4 /dev/sda
> $ mount /dev/sda /mnt/discardtest
> 
> # put some data on it
> $ dd if=/dev/urandom of=/mnt/discardtest/dummy.bin bs=1M count=1000
> $ sync
> 
> # on host, check size...it should be about 1GB
> $ du -sh disk.qcow2
> 
> # in vm, delete file and trim it
> $ rm /mnt/discardtest/dummy.bin
> $ sync
> $ fstrim /mnt/discardtest
> 
> # on host, check size...should only be a few megabytes
> $ du -sh disk.qcow2
> 
> 
> It works with other guest OSes as well, for Windows you just use the
> Optimize-Volume cmdlets instead of fstrim.
> 
> You also don't have to use qcow2 for backing. Anything that can handle
> discards will do, including sparse files.
> 
> 
> My advice, however, would be to use qcow2, since sparse files don't work
> particularly well. Executing the example above would leave you with
> about 300MB at the end instead of less than 10MB.
> 
> Regards
> Friedrich
> 
> On 4/5/19 4:04 PM, Narcis Garcia wrote:
>> No; I want to study the possibility of recovering allocated blocs of
>> host filesystem when guest filesystem removes its files (unallocates its
>> blocks).
>> Host -> HD or SSD (independent) with sparse-mode image
>> Guest -> Virtual SSD (to signal discards)
>>
>>
>> El 5/4/19 a les 16:01, Pascal ha escrit:
>>> hello,
>>>
>>> if I understand correctly, you want to study the possibility of
>>> recovering deleted files from an SSD disk: is that right ?
>>>
>>> regards, lacsaP.
>>>
>>> Le jeu. 4 avr. 2019 à 08:24, Narcis Garcia <address@hidden
>>> <mailto:address@hidden>> a écrit :
>>>
>>>      Is there a way to specify a disk to be detected as an SSD drive?
>>>
>>>      Once reached this, I want to look for the possibility to recover
>>> host
>>>      space when a guest discards disk image blocks, and this image is
>>> RAW
>>>      format and sparse allocated file.
>>>
>>>      Thank you.
>>>
>