Place the first PE at 1 MiB for all defaults

. When using default settings, this commit should change
  nothing.  The first PE continues to be placed at 1 MiB
  resulting in a metadata area size of 1020 KiB (for
  4K page sizes; slightly smaller for larger page sizes.)

. When default_data_alignment is disabled in lvm.conf,
  align pe_start at 1 MiB, based on a default metadata area
  size that adapts to the page size.  Previously, disabling
  this option would result in mda_size that was too small
  for common use, and produced a 64 KiB aligned pe_start.

. Customized pe_start and mda_size values continue to be
  set as before in lvm.conf and command line.

. Remove the configure option for setting default_data_alignment
  at build time.

. Improve alignment related option descriptions.

. Add section about alignment to pvcreate man page.

Previously, DEFAULT_PVMETADATASIZE was 255 sectors.
However, the fact that the config setting named
"default_data_alignment" has a default value of 1 (MiB)
meant that DEFAULT_PVMETADATASIZE was having no effect.

The metadata area size is the space between the start of
the metadata area (page size offset from the start of the
device) and the first PE (1 MiB by default due to
default_data_alignment 1.)  The result is a 1020 KiB metadata
area on machines with 4KiB page size (1024 KiB - 4 KiB),
and smaller on machines with larger page size.

If default_data_alignment was set to 0 (disabled), then
DEFAULT_PVMETADATASIZE 255 would take effect, and produce a
metadata area that was 188 KiB and pe_start of 192 KiB.
This was too small for common use.

This is fixed by making the default metadata area size a
computed value that matches the value produced by
default_data_alignment.

1 files changed, 16 insertions, 15 deletions

diff --git a/tools/args.h b/tools/args.h
index 414d7a84b..522cccfa3 100644
--- a/tools/args.h
+++ b/tools/args.h
@@ -84,14 +84,16 @@ arg(binary_ARG, '\0', "binary", 0, 0, 0,
     "the \"unknown\" value which denotes that the value could not be determined).\n")
 
 arg(bootloaderareasize_ARG, '\0', "bootloaderareasize", sizemb_VAL, 0, 0,
-    "Create a separate bootloader area of specified size besides PV's data\n"
-    "area. The bootloader area is an area of reserved space on the PV from\n"
-    "which LVM will not allocate any extents and it's kept untouched. This is\n"
-    "primarily aimed for use with bootloaders to embed their own data or metadata.\n"
+    "Reserve space for the bootloader between the LVM metadata area and the first PE.\n"
+    "The bootloader area is reserved for bootloaders to embed their own data or\n"
+    "metadata; LVM will not use it.\n"
+    "The bootloader area begins where the first PE would otherwise be located.\n"
+    "The first PE is moved out by the size of the bootloader area, and then moved\n"
+    "out further if necessary to match the data alignment.\n"
     "The start of the bootloader area is always aligned, see also --dataalignment\n"
-    "and --dataalignmentoffset. The bootloader area size may eventually\n"
-    "end up increased due to the alignment, but it's never less than the\n"
-    "size that is requested. To see the bootloader area start and size of\n"
+    "and --dataalignmentoffset. The bootloader area may be larger than requested\n"
+    "due to the alignment, but it's never less than the requested size.\n"
+    "To see the bootloader area start and size of\n"
     "an existing PV use pvs -o +pv_ba_start,pv_ba_size.\n")
 
 arg(cache_long_ARG, '\0', "cache", 0, 0, 0,
@@ -173,14 +175,14 @@ arg(configtype_ARG, '\0', "typeconfig", configtype_VAL, 0, 0,
     "Also see \\fBlvm.conf\\fP(5).\n")
 
 arg(dataalignment_ARG, '\0', "dataalignment", sizekb_VAL, 0, 0,
-    "Align the start of the data to a multiple of this number.\n"
-    "Also specify an appropriate Physical Extent size when creating a VG.\n"
-    "To see the location of the first Physical Extent of an existing PV,\n"
-    "use pvs -o +pe_start. In addition, it may be shifted by an alignment offset.\n"
-    "See lvm.conf/data_alignment_offset_detection and --dataalignmentoffset.\n")
+    "Align the start of a PV data area with a multiple of this number.\n"
+    "To see the location of the first Physical Extent (PE) of an existing PV,\n"
+    "use pvs -o +pe_start. In addition, it may be shifted by an alignment offset,\n"
+    "see --dataalignmentoffset.\n"
+    "Also specify an appropriate PE size when creating a VG.\n")
 
 arg(dataalignmentoffset_ARG, '\0', "dataalignmentoffset", sizekb_VAL, 0, 0,
-    "Shift the start of the data area by this additional offset.\n")
+    "Shift the start of the PV data area by this additional offset.\n")
 
 arg(deduplication_ARG, '\0', "deduplication", bool_VAL, 0, 0,
     "Controls whether deduplication is enabled or disable for VDO volume.\n"
@@ -488,8 +490,7 @@ arg(pvmetadatacopies_ARG, '\0', "pvmetadatacopies", pvmetadatacopies_VAL, 0, 0,
     "The number of metadata areas to set aside on a PV for storing VG metadata.\n"
     "When 2, one copy of the VG metadata is stored at the front of the PV\n"
     "and a second copy is stored at the end.\n"
-    "When 1, one copy of the VG metadata is stored at the front of the PV\n"
-    "(starting in the 5th sector).\n"
+    "When 1, one copy of the VG metadata is stored at the front of the PV.\n"
     "When 0, no copies of the VG metadata are stored on the given PV.\n"
     "This may be useful in VGs containing many PVs (this places limitations\n"
     "on the ability to use vgsplit later.)\n")