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

[Narcis Garcia]

A test to check if guest Linux kernel treats vdisk as SSD:
Instead of:
mount /dev/sda1 /ssd
mount
Do this:
mount -o discard /dev/sda1 /ssd
mount

Anyway I don't think 'strings' test must be reflecting real blocks
discard behavior if not:
1. qemu taking image file actions for guest discard signals
2. Raw disk image being sparsed:
dd if=/dev/zero of=/tmp/ssd.disk obs=1M seek=128 count=0


El 9/4/19 a les 16:38, Pascal ha escrit:
> the blocks do not seem to be discarded on a raw format disk: the data
> remains on the disk...
> 
> on host :
> 
> dd if=/dev/zero of=/tmp/ssd.disk bs=1M count=128
> qemu -drive media=disk,file=linux.disk -device virtio-scsi-pci,id=scsi
> -device scsi-hd,drive=hd -drive if=none,id=hd,file=/tmp/ssd.disk,format=raw
> 
> on Linux VM :
> 
> fdisk /dev/sda
>     # create one partition on gpt type...
> fdisk -l /dev/sda
>     Disk /dev/sda: 128 MiB, 134217728 bytes, 262144 sectors
>     Units: sectors of 1 * 512 = 512 bytes
>     Sector size (logical/physical): 512 bytes / 512 bytes
>     I/O size (minimum/optimal): 512 bytes / 512 bytes
>     Disklabel type: gpt
>     Disk identifier: B8D98B8E-7790-4043-8F37-D4D8CA218884
>     Device         Start    End Sectors  Size Type
>     /dev/sda1  2048 262110  260063  127M Linux filesystem
> mkfs.ext4 -L ssd /dev/sda1
> mount /dev/sda1 /ssd
> mount
>     /dev/sda1 on /ssd type ext4 (rw,relatime,data=ordered)
> echo hello world > /ssd/test
> cat /ssd/test
>     hello world
> sync && sysctl -q vm.drop_caches=3
> rm -f /ssd/test
> sync && sysctl -q vm.drop_caches=3
> fstrim /ssd
> umount /ssd
> strings /dev/sda1 | grep 'hello world'
>     hello world
> 
> 
> Le mar. 9 avr. 2019 à 08:36, Narcis Garcia <address@hidden> a écrit :
> 
>> 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.
>>>>>
>>>
>>
>>