Re: write directly to device mapper snapshot origin device ?

["Navin P.S" <navinp1912@gmail.com>]

On Fri, Jul 15, 2016 at 6:14 PM, Zdenek Kabelac <zkabelac@redhat.com> wrote:
> Dne 15.7.2016 v 14:40 Navin P.S napsal(a):
>
>> On Fri, Jul 15, 2016 at 6:05 PM, Zdenek Kabelac <zkabelac@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@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@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.
>>>>
>
>>
>> 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.
>

1,2,3,4 doesn't say you can't write to snapshot-origin ? Does it ?
If you setup the table with dmsetup -r explicitly it means readonly.

Also if direct writes are not supposed to be done, only merging why
not setup readonly and
then create it in read-write during merging.


-- 
-- Navin