Defrag: defragmentation on dedicated device
shaitan opened this issue · comments
Problem
Defragmentation is connected with a lot of disk io activity and it requires 2 * blob_size as a free space on the device. Currently, defragmentation kills disk of backend and all read/write operations with backends are dramatically slowed down. Also free space requirement reserves space on each backend device, as a result we have 2 * blob_size * number of backends reserved but mostly unused space.
Solution
Make defragmentation on dedicated device. Lets make comparison of defragmentation on dedicated device vs backend's device:
| Measurement | dedicated device | backend's device |
|---|---|---|
| disk io | 1 read and 1 write at the backend's device and 3 read and 3 write at the dedicated device | 3 read, 3 write and 1 move at the backend's device |
| free space requirement for one server | 2 * blob_size on dedicated device |
2 * blob_size * number of backends |
Since defragmentation will have most of disk io at dedicated disk, backend's disk will be free for clients requests.