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

[Pascal]

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.
> >>>
> >
>
>