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

[Narcis Garcia]

Okay, more clear report:
$ lsblk -do NAME,ROTA,DISC-GRAN


El 10/4/19 a les 14:12, Pascal ha escrit:
> I would tend to say that "discard" is not related to "ssd" while "rotational"
> is...
> 
> Le mer. 10 avr. 2019 à 14:06, Pascal <address@hidden> a écrit :
> 
>> lsblk -ndo ROTA /dev/sda
>>
>> sorry for the bad copy.
>>
>> Le mer. 10 avr. 2019 à 12:52, Narcis Garcia <address@hidden> a
>> écrit :
>>
>>> Can be this rotational detection be affecting in this case?
>>>
>>> https://lists.openvz.org/pipermail/users/2019-April/007564.html
>>> (Devuan 1.0 VM with distro kernel succeeds with lsblk and fstrim)
>>>
>>> How can I make guest OS to detect device as no rotational or true SSD?
>>>
>>>
>>> El 10/4/19 a les 12:47, Narcis Garcia ha escrit:
>>>> 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.
>>>>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>
>>>
>>>