Re: RAID performance

[Adam Goryachev <mailinglists@websitemanagers.com.au>]

On 15/02/13 12:40, Chris Murphy wrote:
> 
> On Feb 14, 2013, at 6:10 PM, Adam Goryachev
> <mailinglists@websitemanagers.com.au> wrote:
>> 
>> 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