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

[Mikulas Patocka]

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.

Mikulas