= Disk Alignment =

We use a complex stack of block device tools.  As it turns out, these tools will perform slightly better if they all agree on block boundaries.

== setting up ==

here are the things to think about when setting up a disk:

 * physical disk partitions -- should start at a multiple of 8192s (where s means "512Byte sector")
 * RAID -- mdadm should use 0.90 or 1.0 [https://raid.wiki.kernel.org/index.php/RAID_superblock_formats superblocks], which are written at the end of the device.  If metadata version 1.1 or 1.2 are used, then the `data_offset` field needs to be a multiple of 8192. [http://www.issociate.de/board/goto/2027709/Alignment_of_RAID_on_specific_boundary.html there does not seem to be a way to force a custom data_offset with metadata 1.1 or 1.2] (i filed [DebianBug:614841] as a wishlist).  in squeeze, 1.2 is the default, so use `--metadata 1.0` during `mdadm --create` :(
 * dm_crypt/LUKS -- `cryptsetup LuksFormat` should use `--align-payload=8192`
 * LVM 
  * `pvcreate` should use `--dataalignment 8192s`
  * `vgcreate` should use `--physicalextentsize 8192s` (this is currently the default)


== checking ==

 * Partitions: (look at Start column -- you want multiples of 8192) `parted /dev/sda unit s print`
 * RAID: `mdadm --examine $COMPONENTDEVICE | grep Version` 
  * if the version is 1.1 or 1.2, then look at the data offset: `mdadm --examine $COMPONENTDEVICE | grep 'Data Offset:` 
 * dm_crypt/LUKS: `cryptsetup luksDump $BASEDEVICE | grep '^Payload offset:'`
 * LVM
  * Physical volumes: (look at "1st PE"): `pvs --units s -o +pe_start`
  * Volume groups: (look at "Ext"): `vgs --units s -o +vg_extent_size`

== fixing ==

if any of these are wrong, what do we do?