[linux-lvm] 2 LVM questions - LVM and 2.4.19, and extent based striping.

[Alasdair G Kergon]

On Tue, Nov 26, 2002 at 09:32:44PM +0000, Alasdair Kergon wrote:
> If you have 18 extents (PEs) with 6 on each disk numbered:
>   disk1:   0  1  2  3  4  5
>   disk2:   6  7  8  9 10 11
>   disk3:  12 13 14 15 16 17
[More on the numbering LVM* uses, in case that wasn't clear enough.]

Depending whereabouts those extents are on the disk, 
the *actual* PE numbers that LVM* assigns could be 
completely different (but still consecutive on each disk
inside each segment of the LV) e.g.
    disk1:   0  1  2  3  4  5
    disk2:  50 51 52 53 54 55
    disk3:   8  9 10 11 12 13

And the corresponding logical extent (LE) numbers within the 
logical volume run *along* each stripe of the segment in turn:
    disk1:   0  1  2  3  4  5
    disk2:   6  7  8  9 10 11
    disk3:  12 13 14 15 16 17
So in LE terms, the data get striped across 0, 6 and 12, then 
1, 7 and 13 etc., equivalent to striping across 3 extents each
6 times bigger, just one per disk viz.  [0..5], [6..11] and [12..17].
 
Alasdair
-- 
agk at uk.sistina.com