[Bug 756853] Re: 11.04 beta aligns 4k disks on cylinder 63

[alp]

Hi Colin,

I can confirm that the Ubuntu 11.04 64-bit desktop release continues to
exhibit this issue, and I have installer logs pinpointing exactly when
this happened. The installation was done on a MacBook Pro with a WDC
WD7500BPKT-75PK4T0 750 GB hard drive featuring Advanced Format 4096
blocks:

# cat /sys/block/sda/queue/physical_block_size 
4096
# cat /sys/block/sda/queue/logical_block_size 
512

Before starting, Mac OS X Snow Leopard was allocated 650 GB with the
remaining 100 GB unallocated. I selected the automatic single click
partitioning option in the graphical desktop Ubuntu installer assuming
it would create boot, root and swap partitions in the remaining space,
which it did without any intervention.

However, while the boot and swap partitions it created were
appropriately aligned, the main root partition (number 4) the installer
created was unfortunately not aligned. This is a printout from gdisk:

Disk /dev/disk0: 1465149168 sectors, 698.6 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): ***
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 1465149134
Partitions will be aligned on 8-sector boundaries
Total free space is 86 sectors (43.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          409639   200.0 MiB   EF00  EFI System Partition
   2          409640      1269940895   605.4 GiB   AF00  Untitled 1
   3      1269940896      1269942849   977.0 KiB   EF02  
   4      1269942850      1456800271   89.1 GiB    0700  
   5      1456800272      1465149054   4.0 GiB     8200  


As you can see, partition 4, the main Linux ext4 partition is not aligned.

The Ubuntu disk utility has also identified this issue (screenshot with
my serial number redacted attached):

WARNING: The partition is misaligned by 3072 bytes. This may result in
very poor performance. Repartitioning is suggested.

Excerpt from the partman log where it created the misaligned partition:

parted_server: Opening outfifo
parted_server: requested partition with type primary
parted_server: requested partition with file system ext4
parted_server: add_primary_partition(disk(1465149168),1269942850-1456800271)
parted_server: OUT: OK


I've no idea if this might have happened because the hardware or kernel mis-reported the optimal alignment, or if it's some other issue, but I would assume that the easy solution is to enforce 4096 alignment on all drives when partitioning automatically, as the benefit outweighs the potential loss of a megabyte or two.

Let me know if you need the entire partman installer log or any other
details. Also, please let me know if you find the issue so I can wipe
the misaligned partition and re-install, as I'm only keeping it around
to help diagnose this bug.


** Attachment added: "Misalignment warning screenshot"
   https://bugs.launchpad.net/ubuntu/+source/partman-base/+bug/756853/+attachment/2163039/+files/screenshot-misaligned-redacted.png

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https://bugs.launchpad.net/bugs/756853

Title:
  11.04 beta aligns 4k disks on cylinder 63

Status in “partman-base” package in Ubuntu:
  Incomplete

Bug description:
  Binary package hint: ubiquity

  The default install partitions disks with 4k sectors improperly. When
  electing to use the entire disk, the primary partition is misaligned
  on newer systems. On my disk, the partitions were aligned at cylinder
  63. The automatic installer should either check the reported sector
  size (not always reliable), or leave unallocated space at the front of
  the drive to ensure that newer disks with 4k sectors, as well as older
  drives with 512-byte sectors, are aligned properly.

  The only workaround I could find was to manually allocate *all*
  partitions. To do that seems to require two full trips through the
  installer, because manual partitions are only written during final
  installation--it doesn't appear that there's a way to create a 1MB
  partition and then allow Ubuntu to install the rest automatically
  unless the empty space is detected before the partitioning section of
  the installer.

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