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@xxxxxxxxx> 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, -- NavinHi 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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
