[linux-lvm] Data alignment

Stuart D. Gathman stuart at bmsi.com
Sat Mar 20 23:02:56 UTC 2010

On Fri, 19 Mar 2010, Phillip Susi wrote:

> I have been trying to get my logical volume to be aligned to a 512 KB
> boundary on my SSD so it lines up with the flash erase block.  I thought
> I could do this with pvcreate --dataalign 1024, but this caused the data
> to be aligned to a 1024 sector boundary within the partition, which
> itself starts on sector 63.  I then thought the --dataalignoffsest
> switch would help, but specifying 63 there caused the data to be moved
> to sector 1087.  I tried giving it -63, but was told that a negative
> number is not allowed.  Why not?  How can I get the data to start on
> sector 1024-63 of the partition so it ends up on pysical sector 1024 of
> the disk?

You can have "negative" offsets.  Just subtract from the modulus.  In your
case that would be 1024-63.  I.e. --dataalignoffset 961

If the PV is not removeable, you can skip the partition table and use the
entire device for the PV - leaving 63 more sectors for metadata.

If the PV is removeable, remember you need vgchange -ay vgname after 
attaching, and vgchange -an vgname before removing.  (I would suggest
leaving the partition table if removeable so that other systems will
recognize the partition label.)

	      Stuart D. Gathman <stuart at bmsi.com>
    Business Management Systems Inc.  Phone: 703 591-0911 Fax: 703 591-6154
"Confutatis maledictis, flammis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.

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