[CentOS] ext4, 4k sector alignment

Andrzej Szymanski szymans at agh.edu.pl
Tue Jul 26 08:11:57 UTC 2011

On 2011-07-25 19:10, Les Mikesell wrote:
> 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.

I think so. Your partition starts at sector 63, you need anything 
divisible by 8, so:
- 64 is my expectation,
- 56 is a fallback solution if the partition does not fit on the disk 
with 64 sector offset
- 2048 would be perfect (1M alignment is currently preferred by Centos 6 
and many other OSs)

To be on the safe side, take the disk out of the array (mdadm -f 
/dev/md0 /dev/sdX1 ; mdadm -r /dev/md0 /dev/sdX1) and clear superblock 
using mdadm --zero-superblock /dev/sdX1.

Then repartition the disk using fdisk using the following commands:
fdisk /dev/sdX
u   -- display units are sectors
c   -- no DOS compatibility (== no cyllinder rounding, you definitely 
want that)
o   -- new dos partition table
n   -- new partition
p   -- primary
1   -- partition 1
64  -- starting offset
1465144065 -- exact size here, because (just to be on the safe side) you 
do not want to have a larger partition on a rescue disk than on a base 
disk. Your partition sdh1 has 732572001 1k-blocks, as you wrote in one 
e-mail, multiply this by 2 (sectors) add 64, (starting offset) subtract 
1 because the offset is inclusive. You get 2*732572001+64-1=1465144065. 
If fdisk complains that this is too much then offset 64 cannot be used 
and you need to repeat the procedure using offset 56 (don't forget to 
recalculate ending sector).
t   -- type
fd  -- linux raid autodetect

mdadm -a /dev/mdX /dev/sdX1

And everything should be fine.

> Is it worth converting to ext4?

I don't know.

> Is there a difference between doing this on 5.6 or 6.x?

Yes, Centos 6 by default aligns partitions on the disk automatically on 
1MiB (2048sectors) boundaries. C6 LVM also aligns lv's on 1MiB 
boundaries relative to the pv start. Finally md in Centos 6 uses 512KiB 
chunks and aligns data on this boundary (default md superblock in Centos 
6 Installer is 1.1 so it is on beginning of the partition) so it is also OK.

> If I start over from scratch with 6.x, will the partitioning tools
> automatically align for 4k sector drives (with/without lvm?)?

Yes. But I always check that to be sure :)


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