[Trac-bugs] [PC-BSD Trac] #741: 4k partion discovery scripts seem not to work right.

PC-BSD trac at pcbsd.org
Sat Aug 3 09:59:43 PDT 2013


#741: 4k partion discovery scripts seem not to work right.
---------------------------+------------------
 Reporter:  sara           |      Owner:  kris
     Type:  System Defect  |     Status:  new
 Priority:  minor          |  Milestone:
Component:  Installer      |    Version:
 Keywords:  9-STABLE-p2    |
---------------------------+------------------
 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:
 https://github.com/zfsonlinux/zfs/issues/967
 ---------------------------------------
 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

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Ticket URL: <http://trac.pcbsd.org/ticket/741>
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