[linux-lvm] Data alignment

[Phillip Susi]

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?