[nSLUG] Advanced Format Disk (was: Best way to upgrade mirrored disks to larger capacity?)

D G Teed donald.teed at gmail.com
Thu May 19 11:58:16 ADT 2011

On Thu, May 19, 2011 at 8:22 AM, George N. White III <gnwiii at gmail.com>wrote:

> On Wed, May 18, 2011 at 11:35 PM, D G Teed <donald.teed at gmail.com> wrote:
> > [...]
> > Partitioning has come up as an issue I was not aware of before.  They
> > require
> > partitions to be on certain boundaries with the Western Digital "Advanced
> > Format"
> > and the move to 4KB blocksize.  Western Digital's info on it for Linux is
> > rather lean.
> > I found a nice page telling us how to do the math in fdisk and
> illustrating
> > the performance difference in having the partitions sitting at the right
> > boundaries.
> >
> > http://linuxconfig.org/linux-wd-ears-advanced-format
> >
> > Note how when using multiple partitions, the next start block must be 8
> > greater
> > than the previous end block.  This becomes a gap of 64 with logical
> > partitions,
> > including a gap of 64 prior to the first partition in the extended
> > partition.
> >
> > Is there something like this happening with Seagate or others?
> Yes -- "Advance Format" is an industry-wide initiative.
> <http://ubuntuforums.org/showthread.php?t=1619186> indicates that
> Ubuntu 10.04 and
> later should handle Advance Format disks, and has a link to a nice
> DeveloperWorks article:
> <http://www.ibm.com/developerworks/linux/library/l-4kb-sector-disks/>
> See <http://en.wikipedia.org/wiki/Advanced_Format>, which says:
> "When using Advanced Format drives with legacy operating systems, it
> is important to realign the disk drive using software provided by the
> hard disk manufacturer. Disk realignment is necessary to avoid a
> performance degrading condition known as cluster straddling where a
> shifted partition causes filesystem clusters to span multiple physical
> disk sectors. Since cluster to sector alignment is determined when
> creating hard drive partitions, the realignment software is used
> "after" partitioning the disk. This can help reduce the number of
> unaligned writes generated by the computing ecosystem. "
> In particular: <http://lwn.net/Articles/377897/> discusses the
> implications for linux.  Most of the problems appear to come when
> Windows XP partitions are needed on a disk shared with linux.

Linux has problems all on its own.  fdisk found in Debian squeeze
defaults to creating the first partition at sector 63, violating
the divisible by 8 rule.  I don't know what the installers from
various distros do.

Western Digital's site makes it sound like things are
well under control in any modern Linux distro, but
my experience with the defaults of gparted or fdisk
shows this isn't  what you get.  You need to manually
control and not accept defaults.

The IBM article states gparted must be used with
"Round to cylinders" option unchecked (not intuitive at all),
and if using fdisk, the flags -H 224 -S 56 are required if you
are not going to verify all the numbers manually in fdisk
run without these options.

I just tried fdisk with these funky options, creating
an 8GB partition for sdb1 and got this result (fdisk -lu ):
   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1              56    16783871     8391908   83  Linux

The start sector defaulted to less than 64.  I thought perhaps it
would also choose an end sector divisible by 8.

gparted no longer has "Round to Cylinders", but now shows
"Align to:" with options MiB, Cylinder and None.  I selected MiB,
provided the first partition with 2 MiB gap before it starts, and added
a second partition of swap without telling gparted to leave a gap
( to see if it adds the 8 sectors as advised in the linuxconfig.org
article).  The results were not the best:

fdisk -lu /dev/sdb

Disk /dev/sdb: 1000.2 GB, 1000204886016 bytes
255 heads, 63 sectors/track, 121601 cylinders, total 1953525168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0001dd4d

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            4096    16388095     8192000   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sdb2        16388096    22532095     3072000   82  Linux swap / Solaris

Looks like I will be doing this manually as per the linuxconfig.org article,
or something.

fdisk has a -b option for blocksize.  If I set that to 4096 and include
the -H 224 -S 56 options, I get "Floating point exception" when
making a new primary partition (after the first prompt for partition

I don't see any articles suggesting the larger blocksize be set up
within fdisk with -b.  Would have thought this would match.

There are a lot of articles out there saying different things. Some people
-c on fdisk is all you need.

Thankfully when we chuck a bunch of these into hardware RAID
and use them they are using the whole disk and we don't have
this step.
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