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

[Narcis Garcia]

Pascal, I don't understand syntax you use for lsblk.
I see this with:
$ cat /sys/block/sda/queue/rotational


El 10/4/19 a les 12:38, Pascal ha escrit:
> it's ok with  discard=unmap  option (even if the disc is thick-provisioned)
> : the block used by the  test  file appears (VM side) or is reset (host
> side) at zero.
> 
> notice that  lsblk  "thinks" that the disc is a rotational disk (eg. not
> really SSD) :
> 
> lsblk -ndo /dev/sda
> 1
> 
> thanks for explanations !
> 
> Le mar. 9 avr. 2019 à 20:38, Narcis Garcia <address@hidden> a écrit :
> 
>> I see now the simple:
>> -device virtio-scsi-pci
>> makes all -disk (media=disk) to be detected by guest as SSD
>>
>> Additionally, adding "discard=unmap,detect-zeroes=unmap" to disk image
>> specification makes host Qemu to discard sparse image holes and recover
>> host disk space when guest sends discard signal!
>>
>> This is my successful test with Debian 9 (stable):
>>
>> $ dd if=/dev/zero of=ssd.img obs=1M seek=128 count=0
>> $ qemu-system-... -device virtio-scsi-pci -drive
>> file=ssd.img,media=disk,index=0,discard=unmap,detect-zeroes=unmap
>>
>> Thank you everyone.
>>
>>
>> El 9/4/19 a les 19:19, Friedrich Oslage ha escrit:
>>> 1. You need to set the discard option for the -drive parameter, eg
>>> -drive if=none,...,format=raw,discard=unmap since the default is still
>>> to simply ignore discards.
>>>
>>> You may also want to set the detect-zeroes option to unmap, to discard
>>> all-zero blocks instead of actually writing them.
>>>
>>>
>>> 2. The disk image you created is thick-provisioned. How is qemu supposed
>>> to discard anything in that file? It could turn it into a sparse file or
>>> overwrite it with zeroes I suppose, but both options are undesirable.
>>>
>>> To make use of the discard operation your backing storage has to support
>>> it. Either by beeing thin-provisioned, like qcow2 or raw sparse files,
>>> or by actually beeing a ssd/nvme disk.
>>>
>>>
>>> 3. Your testing method is flawed. You are using a journaling filesystem
>>> to write the data but to read it you access the block device directly .
>>> Even if the discard operation was working strings(1) would still show
>>> the hello-world string in the journal.
>>>
>>> You could try something like this:
>>>
>>> $ echo hello, world > /dev/sda
>>> $ strings /dev/sda
>>> $ blkdiscard /dev/sda
>>> $ strings /dev/sda
>>>
>>>
>>> Regards
>>> Friedrich
>>>
>>> On 4/9/19 4:38 PM, Pascal wrote:
>>>> 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.
>>>>>>>>
>>>>>
>>>
>>
>>