[dm-devel] Snapshot origin table as P/PO snapshot with /dev/null

[Zdenek Kabelac]

On 26.5.2016 16:12, Mikulas Patocka wrote:
>
>
> On Thu, 26 May 2016, Navin P.S wrote:
>
>> Hi, Can you please help me with the below query that i posted to dm-devel ?
>>
>> On Thu, May 26, 2016 at 5:07 AM, Navin P.S <navinp1912 at gmail.com> wrote:
>>       Hi,  If i have a snapshot origin created using dmsetup create table with a mapping  real device  with (major,minor) =(8,18) and lets name it /dev/mapper/dmso
>>       . Now the set of reads are writes done from /dev/mapper/dmso have some values (R,W).
>>
>> Had i been creating a persistent snapshot using (P or PO) mode with origin device as /dev/null and COW device is my real device (maj,min)=8,18 and it is named as
>> /dev/mapper/dmsp. Now if i do the exact same reads and writes that i did to dmso (snapshot-origin) , will get i the same set of output values (R,W).
>>
>> In my experiment i did. So i'm asking is there a case where it differs ?
>>
>> Also i looked at the code in drivers/md/dm-snap.c and i find that both the drivers snapshot and snapshot-origin use the same __orgin_write . So my
>> partial/incomplete understanding is that it works from the read/write view point from /dev/mapper/dmsp.
>>
>> I'll not be using the snapshot-merge feature , so merging to /dev/null is irrelevant here.
>>
>> Regards,
>> -- Navin
>
> Hi
>
> The snapshot target doesn't accept /dev/null because it requires a block
> device and /dev/null is chatected device.
>
> You can use the "zero" device mapper target to create a block device that
> returns zeroes on reads and that discards writes. You can use this zero
> device as an origin to the dm-snapshot target.
>
> With LVM, you can use this functionality with lvcreate --virtualsize
> switch.
>


Just a minor clarification - if you have any 'recent' version of lvm2
this functionality is handled via thin-provisioning by default.

So if you really want to use 'old' snapshot target - just use:

lvcreate --type snapshot -Lrealsize  -Vvirtualsize  vg


And if you want to see exact ioctl sequencing - then use lvcreate -vvvv.

Regards

Zdenek