Change default alignment of pe_start to 1MB.

The new standard in the storage industry is to default alignment of data
areas to 1MB.  fdisk, parted, and mdadm have all been updated to this
default.

Update LVM to align the PV's data area start (pe_start) to 1MB.  This
provides a more useful default than the previous default of 64K (which
generally ended up being a 192K pe_start once the first metadata area
was created).

Before this patch:
# pvs -o name,vg_mda_size,pe_start
  PV         VMdaSize  1st PE
  /dev/sdd     188.00k 192.00k

After this patch:
# pvs -o name,vg_mda_size,pe_start
  PV         VMdaSize  1st PE
  /dev/sdd    1020.00k   1.00m

The heuristic for setting the default alignment for LVM data areas is:
- If the default value (1MB) is a multiple of the detected alignment
  then just use the default.
- Otherwise, use the detected value.

In practice this means we'll almost always use 1MB -- that is unless:
- the alignment was explicitly specified with --dataalignment
- or MD's full stripe width, or the {minimum,optimal}_io_size exceeds
  1MB
- or the specified/detected value is not a power-of-2

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/example.conf.in b/doc/example.conf.in
index 73832d98..a107d2bf 100644
--- a/doc/example.conf.in
+++ b/doc/example.conf.in
@@ -113,7 +113,7 @@ devices {
     # Alignment (in KB) of start of data area when creating a new PV.
     # If a PV is placed directly upon an md device and md_chunk_alignment or
     # data_alignment_detection is enabled this parameter is ignored.
-    # Set to 0 for the default alignment of 64KB or page size, if larger.
+    # Set to 0 for the default alignment of 1MB or page size, if larger.
     data_alignment = 0
 
     # By default, the start of the PV's aligned data area will be shifted by