[dm-devel] write directly to device mapper snapshot origin device ?

[Zdenek Kabelac]

Dne 15.7.2016 v 14:40 Navin P.S napsal(a):
> On Fri, Jul 15, 2016 at 6:05 PM, Zdenek Kabelac <zkabelac at redhat.com> wrote:
>> Dne 15.7.2016 v 14:15 Navin P.S napsal(a):
>>
>>> On Fri, Jul 15, 2016 at 5:21 PM, Mikulas Patocka <mpatocka at redhat.com>
>>> wrote:
>>>>
>>>>
>>>>
>>>> On Fri, 15 Jul 2016, Navin P.S wrote:
>>>>
>>>>> Resend ..
>>>>>
>>>>> On Thu, Jul 14, 2016 at 7:55 PM, Navin P.S <navinp1912 at gmail.com> wrote:
>>>>>>
>>>>>> Hi,
>>>>>>   Can i do dd if=/dev/zero of=/dev/mapper/dmo count=20 ? where dmo is
>>>>>> a dm snapshot origin created through dmsetup command. Instead of
>>>>>> /dev/zero i can write valid devices.
>>>>>>
>>>>>> I could create the dmo through some lvm2 commands.
>>>>
>>>>
>>>> lvcreate -s
>>>>
>>>>>> Is direct write
>>>>>> with 4k (page size values ) allowed when it is mapped to 2 block
>>>>>> devices in linear mode ?
>>>>>>
>>>>>>
>>>>>> Regards,
>>>>>> -- Navin
>>>>>
>>>>>
>>>>> I'm seeing  OOM followed by reboot of host machine.
>>>>> Does this mean this device cannot to exported to containers for writing
>>>>> ?
>>>>
>>>>
>>>> The snapshot driver requires 32 bytes of memory for one chunk (the chunk
>>>> size is selected with the "-c" switch to lvcreate). You should set system
>>>> memory according to that.
>>>>
>>>> Mikulas
>>>>
>>>>> -- Navin
>>>>>
>>>
>>>
>>>
>>>
>>> Are origin devices readonly ?
>>>
>>> When i do dd if=/dev/zero of=/dev/mapper/ff count=10, it kills all the
>>> process says OOM.
>>>
>>> How do you go about debugging this ? I'm running 4.7.0-rc7+ .
>>>
>>>
>>> root at vm-xenial-foo:~# dmsetup info
>>> Name:              cryptie4-ff
>>> State:             ACTIVE
>>> Read Ahead:        256
>>> Tables present:    LIVE
>>> Open count:        0
>>> Event number:      0
>>> Major, minor:      251, 0
>>> Number of targets: 2
>>>
>>> Name:              ff
>>> State:             ACTIVE
>>> Read Ahead:        256
>>> Tables present:    LIVE
>>> Open count:        0
>>> Event number:      0
>>> Major, minor:      251, 1
>>> Number of targets: 1
>>>
>>> root at vm-xenial-foo:~# dmsetup status
>>> cryptie4-ff: 0 102400 linear
>>> cryptie4-ff: 102400 122880 linear
>>> ff: 0 102400 snapshot-origin
>>> root at vm-xenial-foo:~#
>>
>>
>> Hi
>>
>> Maybe you should have started with reading documenation for snapshot target
>> ?
>>
>> linux/Documentation/device-mapper/snapshot.txt
>>
>
> It nowhere mentions that origin device is readonly. It says writes go
> to COW device
> and then you can merge it back.
>
> Can you please paste the snippet ?


1) a device containing the original mapping table of the source volume;
2) a device used as the <COW device>;
3) a "snapshot" device, combining #1 and #2, which is the visible snapshot
    volume;
4) the "original" volume (which uses the device number used by the original
    source volume), whose table is replaced by a "snapshot-origin" mapping
    from device #1.

Regards

Zdenek