[linux-lvm] Data alignment

[Mike Snitzer]

On Fri, Mar 19 2010 at  5:27pm -0400,
Phillip Susi <psusi at cfl.rr.com> wrote:

> On 3/19/2010 3:25 PM, Mike Snitzer wrote:
> > Any reason why you'd like to use DOS partitioning (first primary
> > partition starting at 63rd sector)?
> 
> Convention.  I am looking into using GPT instead though.
> 
> > Anyway, if you have a recent kernel (e.g. 2.6.33) you'll be in good
> > shape regardless.  If you create a partition on the device (using DOS
> > partitions) the kernel _should_ be aware of where the partition starts
> > and tell you how far you'd have to shift the start of your LVM2 PV data
> > area to get it perfectly aligned relative to the underlying physical
> > block size.  Check for example:
> > # cat /sys/block/sda/sda1/alignment_offset
> > 
> > But if your device is using 512b physical_sector_size you'll just have a
> > 0 for alignment_offset.  Check physical_block_size with:
> > # cat /sys/block/sda/queue/physical_block_size
> 
> 0 alignment, physical block size 512.
> 
> > Also verify that your SSD device is naturally aligned (aka
> > alignment_offset=0); I'd wager it is naturally aligned:
> > # cat /sys/block/sda/alignment_offset
> 
> Kernel thinks so, and based on performance tests it appears so.
> 
> > All said, even if you have an older kernel, to manually get what you
> > want (shift start to account for DOS partition at 63rd sector, align PV
> > pe_start on a 512K boundary), please try:
> > # pvcreate --dataalignmentoffset 512b --dataalignment 512K ...
> 
> Won't that just add one sector to the start, placing it at sector 1025?
>  How does an alignment offset of 1 sector account for the partition
> starting on sector 63?

The --dataalignmentoffset acts as padding.  I was focused on getting you
to a power of 2 start (sector 64).  Then from there --dataalignment
governs the start boundary for the PV data area.

So you're right, 64*512b + 512K isn't a multiple of 512K :)

This will get you what you want:
# pvcreate --dataalignmentoffset 512b --dataalignment 480K /dev/sda1

# pvs --units s -o+pe_start /dev/sda1
  PV         VG   Fmt  Attr PSize     PFree     1st PE 
  /dev/sda1       lvm2 a-   20971457S 20971457S    961S

961*512b + 63*512b = 512K