4k partion discovery scripts seem not to work right.
|Reported by:||sara||Owned by:||kris|
After reading through some of the scripts from the installer backend, and others, it seemed the installer tries to identify 4k drives then use them correctly. I did not spend a lot of time on them because I was trying to fix my system, but I do have a 512 sector size drive and it setup my disk and zfs with 4k options. I don't know if this will hurt my performance, but it can waste space. I have an older 40GB drive here.
However, we need to be aware that this will cost us LOTS of capacity for certain configurations. A good real world example of this is described in issue #548. In summary, the portage-tree source takes 672M on ext4, but in a 4+2 RAIDZ2 configuration with 4k sectors expands to consume 1.5GB. For this sort of workload ashift=9 would be a much better choice.
Here is the list of issues I'm aware of concerning moving to 4k sectors by default:
Migrating existing vdevs from 512b to 4k
Reduced compression ratios
Blocks <4k will get 0% compression, 8k blocks 50%, etc
Some metadata is allocated using 4k blocks and will never get compressed
Incorrect accounting of compressed dataset sizes
For small blocks RAID-Z degrades to a mirror (or worse in special cases)
Reduced uberblocks for recovery
PC-BSD 9-STABLE-p2 amd64 INSTALL DVD/USB - Fri Jul 19 19:37:02 EDT 2013