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 <[email protected]> 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
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