#741 closed System Defect (fixed)

4k partion discovery scripts seem not to work right.

Reported by: sara Owned by: kris
Priority: minor Milestone:
Component: Installer Version:
Keywords: 9-STABLE-p2 Cc: trac-bugs@…


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.

Some quotes:

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)
Grub support
Reduced uberblocks for recovery

PC-BSD 9-STABLE-p2 amd64 INSTALL DVD/USB - Fri Jul 19 19:37:02 EDT 2013

Attachments (1)

ssd_specs (1.1 KB) - added by sara 22 months ago.

Download all attachments as: .zip

Change History (3)

Changed 22 months ago by sara

comment:1 Changed 22 months ago by sara

This is in the attachment, but I just now realized the installer put my first zfs starting at sector 2088. What is going on with the the alignment?

comment:2 Changed 22 months ago by kris

  • Resolution set to fixed
  • Status changed from new to closed

This is fixed in the latest set of ISO's uploading right now. It'll use 512B blocks by default, unless you enable the option to "Force 4K".

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