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Re: How do I tell if my partition is aligned for a 64k/RAID-6?

To: "Josef 'Jeff' Sipek" <jeffpc@xxxxxxxxxxxxxx>
Subject: Re: How do I tell if my partition is aligned for a 64k/RAID-6?
From: Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx>
Date: Sat, 27 Dec 2008 08:53:21 -0500 (EST)
Cc: Michal Soltys <soltys@xxxxxxxx>, linux-raid@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20081227133824.GH6284@xxxxxxxxxxxxxx>
References: <alpine.DEB.1.10.0812250821360.31386@xxxxxxxxxxxxxxxx> <495410F4.5010508@xxxxxxxx> <alpine.DEB.1.10.0812251846270.32331@xxxxxxxxxxxxxxxx> <20081227133824.GH6284@xxxxxxxxxxxxxx>
User-agent: Alpine 1.10 (DEB 962 2008-03-14)


On Sat, 27 Dec 2008, Josef 'Jeff' Sipek wrote:

On Thu, Dec 25, 2008 at 06:46:37PM -0500, Justin Piszcz wrote:
On Fri, 26 Dec 2008, Michal Soltys wrote:
I am not worried about the filesystem as the defaults usually get it right
but with parted, this is the first time I had to use it for home use and
with RAID-6 I am noticing slower performance with 15 disks (1 is a spare)
in RAID-6 (albeit slower 7200 ones, RE3s) than I was getting with 10
raptor150s in RAID-6 (but I had used fdisk there).

Justin.


For best effect - partition should start at 1920th sector (stripe width
boundary), and su/sw should be appropriately set - su=64k,sw=13 in your
case. Parted shows the values normally - from the beginning of the volume, 0
based.


ps.

I dropped some of CCs.

Thanks.

It looks like you made why big partition. Why bother with partitioning at
all? If you just use the whole device, you _will_ be aligned properly if you
tell mkfs.xfs about the stripe unit/width.

Josef 'Jeff' Sipek.

Why bother partitioning? I also run a Linux SW RAID1 on the host, as you know the drive letters can change when working on a server.

Example: Before I created the array (RAID-6), my disks were /dev/sda, /dev/sdb and /dev/sdc (spare), the RAID-6 was /dev/sdd. After reboot, /dev/sdd became /dev/sda and /dev/sd[a-c] became /dev/sd[b-d].

In lilo.conf I had:
raid-extra-boot="/dev/sda,/dev/sdb"

If I ran LILO and forgot to fix the lilo.conf to reflect the new drive mappings:
raid-extra-boot="/dev/sdb,/dev/sdc"

I could be at risk of ruining the partition?

I try to avoid such situations when possible, to be safe.

Justin.


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