[Discuss] ZFS for an Encrypted External Disk: My Experience and Notes

[Kent Borg]

On 7/25/22 08:58, Rich Pieri wrote:
>      zfs set copies=2 extern_backup_pool

Of my two new ZFS disks, I think have converted one of them over to "copies=2" with the data all in double copies. And that took quite some time!

As fast as USB-C is, as fast as modern spinning rust is, copying around 800GB of data, for the side effect of taking up 1.2GB of disk space, is a lot of data movement.

It took hours. The fact that ZFS assumes that numerically distant LBAs will be physically distant, to the extent that assumption holds, will make for a lot of extra head movement. Maybe Linux can reorder writes around that a little, but it is a pessimizing thing to do for speed. (Issuing parallel writes to two different drives suddenly seems like such a luxury.)

I then did my regular incremental backup (my own script, basically a packaging of rsync's cool "link-dest" feature) and it took…about the same amount of time as before. Which makes sense, most of the work is checking whether a file has changed, and therefore whether it needs a backup. The actual writing of new data to the "copies=2" disk is not so great, so the extra write is a small component of the total time.

Were the disk I'm backing up also ZFS, it could be done much more efficiently and quickly and precisely, but by doing it this old fashioned rsync way I/do/  get the side effect of a partial, kinda half-scrub, specifically of the data that is of current interest.

-kb