[CentOS] ext4, 4k sector alignment
incoming-centos at rjl.com
Tue Jul 26 05:31:45 UTC 2011
On 07/25/2011 10:10 AM, Les Mikesell wrote:
> I've mentioned this problem before but put off doing anything about it
> and maybe now someone can suggest the best solution.
> I have a 3-member RAID1 set where one of the members is periodically
> swapped and rotated offsite. The filesystem contains a backuppc archive
> which has millions of hardlinks that make it impractical to copy with a
> file-oriented approach. The current filesystem is ext3 with one
> partition that uses the entire disk capacity (no lvm). It works as is,
> I'd like to use a laptop size drive for the swapped member and the only
> ones available that match the size have 4k sectors. I have swappable,
> trayless SATA bays available for both drive sizes. The problem is that
> with the current partition layout, the drive with 4k sectors takes more
> than a day to re-sync even though on read access the speed is a match
> for the full sized drives that sync in a few hours.
> My questions for any filesystem experts are:
> Is there a way to adjust the existing md partitions to get the right
> alignment for 4k sectors without having to do a file-oriented copy to
> new partitions? A resize + a dd copy to shift the position might be
> feasible time-wise if that would work.
> Is it worth converting to ext4?
> Is there a difference between doing this on 5.6 or 6.x?
> If I start over from scratch with 6.x, will the partitioning tools
> automatically align for 4k sector drives (with/without lvm?)?
For LVM's see the --dataalignment and --dataalignmentoffset options.
For md devices, my understanding is that the raid superblock is at the
end of the partition, so the data is aligned with wherever the partition
starts. I verified this using:
hexdump /dev/md1 | head -6
hexdump /dev/sda4 | head -6
More information about the CentOS