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Re: [linux-lvm] Volume alignment over RAID



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?
run:
pvs -o pv_name,pe_start

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