On 14/05/11 15:27, Drew wrote:
I'm sure that LV's could be defragmented - there is already code to move
them around on the disks (such as to move them out of a PV before deleting
the PV). I don't know why it hasn't been implemented - maybe there are too
few people working on LVM, or that it is a low priority, or that LV
fragmentation makes very little measurable difference in practice.
I've always figured it was because fragmentation in the LV's caused
little performance degradation. If we were talking about LV's composed
of hundreds of fragments I would expect to see degradation but I've
never come across a scenario where LV's have been that bad.
I too think the fragmentation is probably a small effect - but it is
maybe a measurable effect nonetheless. I've never seen any benchmarks
on it. However, I know that some filesystems (such as xfs in
particular, and ext4 to a lesser extent) go out of their way to reduce
the risk of fragmentation in files - if it is worth their effort, then
perhaps it is also worth it for LVM.
Someone refered to DOS in an earlier post and I think that's a good
example of relevance. I maintain a bunch of Windows based machines at
work and I did some performance benchmarking between a traditional
defrag utility and some of the "professional" versions. Bells and
whistles aside, what set most of the Pro versions apart from the
standard defrag utilities was the concept of "good enough" defrag,
which basically puts files into several larger fragments as opposed to
a complete defrag. I ran tests on filesystem performance before and
after defraging drives with both options and the change in performance
between a full defrag and a "good enough" defrag was minimal.
You will probably also find that the real-world difference between no
defrag and "good enough" defrag is also minimal.
There are some heavily used files and directories that get so badly
fragmented in windows systems that they can benefit from a defrag - for
example the registry files, the windows directory, and some of the NTFS
structures. Of course, these are the parts that normal defrag utilities
can't help - they can't be defragged while the system is running. But
for most other parts of the system, defrag makes very little real
difference, especially as it is so temporary.
In the old days, before DOS and Windows had any sort of file or disk
cache, defraging had a bigger effect. But now you are far better off
spending money on some extra ram for more cache space than on
"professional" defrag programs.
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