[linux-lvm] Volume alignment over RAID

Luca Berra bluca at comedia.it
Fri May 21 05:10:21 UTC 2010

On Thu, May 20, 2010 at 02:24:19PM -0700, Linda A. Walsh wrote:
> I'm a bit unclear as to where some units are applied in my RAID setup, but
> was wondering how LVM interacted, could be, or should be setup so that
> created volumes would be aligned properly on top of a RAID disk.
note that if your rig uses fairly recent software data alignment should
happen automagically.

> I'm using a RAID 'chunk' size of 64k as suggested by the RAID documentation
> and am using 6 disks to create a RAID6, giving 4 units of data/stripe.  Does
I suppose by raid you mean md, so i wonder what documentation you were
looking at?
I think 64k might be small as a chunk size, depending on your array size
you probably want a bigger size.

Then, since with a six drive raid 6 stripe size is always a power of 2,
answers are easy :)

> this mean my logical volume needs to be aligned on a 64K boundary, or a 256k
> boundary? I.e. does 64k usually specify chunk/unit, or chunk/stripe?
align to stripe size

> What do I need to do to make sure my logical volumes always line up on RAID
> stripe boundaries?
make the volume group with pe size multiple of stripe size

> I've been using default logical volume parameters, which I think use an
> allocation size measured in Megabytes, so does that imply I'm automatically
> aligned (as 64k and 256k both divide into 1 Meg)?  Or is some offset
> involved?
pvs -o pv_name,pe_start

Luca Berra -- bluca at comedia.it
         Communication Media & Services S.r.l.
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