[nSLUG] Best way to upgrade mirrored disks to larger capacity?

George N. White III gnwiii at gmail.com
Thu May 19 08:22:14 ADT 2011

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:

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.

George N. White III <aa056 at chebucto.ns.ca>
Head of St. Margarets Bay, Nova Scotia

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