[linux-lvm] The feasibility of implementing an alternative snapshot approach

[Zhiyong Ye]

Dear all,

The current standard lv implementation of snapshots is COW 
(Copy-on-write), which creates snapshots very quickly. However, the 
first write performance of the original lv will be poor after creating a 
snapshot because of COW. Moreover, the more snapshots there are, the 
worse the performance of the original lv will be.

I tested the random read/write performance when the original lv was 
created with different number of snapshots. The data is shown below:
Number of snapshots  Randread(iops)  Randwrite(iops)
         0                21989           22034
         1                10048           10041
         2                6770            6773
         3                5375            5378

There are scenarios where the performance of the original lv is more 
demanding, and the speed of snapshot creation is not as strong a 
requirement. Because it is the original lv that will actually be used, 
and the snapshot is only a secondary function. Therefore snapshots using 
the COW approach will not meet the needs of this scenario.

Therefore, is it feasible to implement another way of taking snapshots? 
Let's say the first snapshot is created as a full snapshot, and all 
subsequent snapshots are based on incremental data from the previous 
snapshot.

Regards,

Zhiyong Ye