On 15/02/13 12:40, Chris Murphy wrote:

On Feb 14, 2013, at 6:10 PM, Adam Goryachev
[off-list ref] wrote:

quoted

So assuming I don't mind wasting a few MB per disk (I was leaving
empty space at the end of the partition anyway), what would I need
to instruct fdisk and/or md to do to get the alignment right?

Current partition/disk is as follows: /dev/sdc1              63
931769999   465884968   fd  Lnx RAID auto

Yeah the old fdisk starts at LBA 63 which is not good because it's
neither 4K nor 8K aligned. For a long time now fdisk starts new ones
at 2048 which is 1MB aligned (and is of course also 8KB and 4KB
aligned, among others so it's just easier to do that.) But for 4K and
8K alignment, start LBA 48, 64, 80, etc are also valid.

Probably a stupid question, but how do I force fdisk to start at 64 ? or
even 2048?

Would it be a sequence like this:
fdisk /dev/sdb
d <- delete the existing partition
u <- change units
n <- new partition
p <- primary
1 <- partition 1
64 <- start sector 64
xxx <- end size of partition

Will that make it right?

I'm planning on doing:
mdadm --manage /dev/md0 --fail /dev/sdb1
mdadm --manage /dev/md0 --remove /dev/sdb1
fdisk (delete and re-create partition, same size, different start/end)
mdadm --manage /dev/md0 --add /dev/sdb1
Wait for resync to complete, and repeat with the next disk.

I was thinking to also use smartctl to instruct the disk to do a format
before creating the new partition. Would that help to "trim" the disk,
or not make any difference at all (since md will re-write the whole disk
in the resync phase anyway).

Thanks,
Adam

-- 
Adam Goryachev
Website Managers
www.websitemanagers.com.au