Comment 4 for bug 1729044

The above usecase was with 1.7 TB ( ~1700GB), To simplify I tried to recreate using the 50 GB just to show the locked memory usage by cinder-backup.

[root@pvc180 cinder]# cinder --service-type volume backup-create --name chec-backup b0f5eec3-1e25-4b2e-8b91-f947b9361dfc
+-----------+--------------------------------------+
| Property | Value |
+-----------+--------------------------------------+
| id | 4d2a21e4-63e7-4018-a5ba-5c79bdcbfee1 |
| name | chec-backup |
| volume_id | b0f5eec3-1e25-4b2e-8b91-f947b9361dfc |
+-----------+--------------------------------------+
[root@pvc180 cinder]# cinder --service-type volume backup-list
+--------------------------------------+--------------------------------------+----------+-------------+------+--------------+-------------------------- ------------------+
| ID | Volume ID | Status | Name | Size | Object Count | Container |
+--------------------------------------+--------------------------------------+----------+-------------+------+--------------+-------------------------- ------------------+
| 4d2a21e4-63e7-4018-a5ba-5c79bdcbfee1 | b0f5eec3-1e25-4b2e-8b91-f947b9361dfc | creating | chec-backup | 50 | 0 | 4d/2a/4d2a21e4-63e7-4018- a5ba-5c79bdcbfee1 |
+--------------------------------------+--------------------------------------+----------+-------------+------+--------------+-------------------------- ------------------+

Memory consumption by cinder-backup
top - 04:49:27 up 1 day, 3:57, 6 users, load average: 1.96, 1.69, 1.24
Tasks: 1 total, 1 running, 0 sleeping, 0 stopped, 0 zombie
%Cpu(s): 3.8 us, 0.4 sy, 0.0 ni, 95.2 id, 0.6 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 15684416 total, 500672 free, 10916160 used, 4267584 buff/cache
KiB Swap: 524224 total, 524224 free, 0 used. 3496064 avail Mem

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23693 cinder 20 0 859008 630592 35584 R 73.7 4.0 13:19.05 cinder-backup

Once the backup completes only half of the memory is released.
top - 04:50:18 up 1 day, 3:57, 6 users, load average: 2.04, 1.76, 1.29
Tasks: 1 total, 0 running, 1 sleeping, 0 stopped, 0 zombie
%Cpu(s): 1.4 us, 0.2 sy, 0.0 ni, 98.4 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 15684416 total, 2085312 free, 10653504 used, 2945600 buff/cache
KiB Swap: 524224 total, 524224 free, 0 used. 3759040 avail Mem

  PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23693 cinder 20 0 600000 372800 35584 S 2.0 2.4 13:49.59 cinder-backup

With huge volumes this impact ends up in 40% of memory being locked.