Re: write directly to device mapper snapshot origin device ?

From: "Navin P.S" <navinp1912@gmail.com>
To: Zdenek Kabelac <zkabelac@redhat.com>
Cc: dm-devel@redhat.com
Subject: Re: write directly to device mapper snapshot origin device ?
Date: Fri, 15 Jul 2016 18:10:00 +0530	[thread overview]
Message-ID: <CAA=4087yxn8zKiHkuGavv4P=8M1DSN3W1c3n_=20+tHVuR=uSw@mail.gmail.com> (raw)
In-Reply-To: <331dd729-34be-a1fa-abfe-c4ad21e7541c@redhat.com>

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.
>>
>> How do you go about debugging this ? I'm running 4.7.0-rc7+ .
>>
>>
>> root@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@vm-xenial-foo:~# dmsetup status
>> cryptie4-ff: 0 102400 linear
>> cryptie4-ff: 102400 122880 linear
>> ff: 0 102400 snapshot-origin
>> root@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 ?