F18 partitioning LVM Question....

[Richard Ryniker]

True, a usage scenario can often be devised to manifest the worst aspects
of any allocation scheme.  There are many other factors that can have so
much influence that I suspect the number of partitions and volume groups
may have little to do with actual performance.

Disk device caching and operating system memory buffer management may
greatly reduce the actual time penalty due to long seeks.

The actual physical extents allocated to a logical volume may be far from
contiguous, therefore long seeks may be needed even for what appears to
be logically contiguous data.

It has to be technically possible, but I have not heard about any utility
to defragment a logical volume... to modify the mapping of logical
extents to physical extents (simultaneously, for all the logical volumes
in a volume group) in order to optimize physical contiguity of logical
data.  This is a very scary thing, because optimum results would require 
understanding and possible adjustment of individual files' allocations
within the file systems in logical volumes.

Disk drives contribute their own distortions into the performance picture
by mapping bad disk blocks to replacement blocks that may be far from the
original block's location.

Conclusion:  this is an interesting topic for discussion, but so many
factors may affect actual results that only careful instrumentation and
measurement of a specific application is likely to show what factors are
significant for that case.

Solid state drives may make these questions of little concern, but raise
some new issues of interest.