On 14/05/11 12:24, Roman Mamedov wrote:
On Sat, 14 May 2011 12:10:56 +0200
David Brown<david.brown@xxxxxxxxxxxx> wrote:
What is perhaps more relevant, is can filesystems see the fragmentation
of the LV's? I don't know the answer.
No, of course they can't.
Still, you don't usually have many segments in an LV - if you want the
LV to be fast, you can request it to be contiguous when creating it.
Then you only get a fragment for each time it is grown. It's a price
often worth paying for the flexibility.
From what I see, the key selling point for LVM is the ability to 'easily'
add/remove/resize LVs. And then if you buy that and start to actively use
these features, you end up in a situation (badly fragmented LVs) from which
there isn't a proper way out. No - backup and restore, or 'have enough
contiguous free space to mirror your entire LV and then nuke the original' are
not the answer. What's sad is that there isn't any fundamental technical
reason LVs can't be defragmented. They can, just no one has bothered to write
the corresponding code yet.
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.
Personally, I find LVM to be a hugely useful tool. I like being able to
make new logical volumes when I need them, and resize them as
convenient. For servers, I make heavy use of openvz lightweight virtual
serving, and I make a new LV for each "machine". So setting up a new
"server" with its own "disk" is done in a couple of minutes. And if the
needs of the "server" outgrow its bounds, it's easy to extend it.
I've had plenty of other cases where LVM has saved me a lot of time and
effort. It's not that long ago since I temporarily needed a bit more
space on a server, and didn't have the time or spare disk to build it
out. So I added a USB disk I had lying around, made a PV on it, and
extended the server's LVs onto the disk. Obviously this sort of thing
gives a performance hit - but it was better to be slow than not working.
For me, LVM's flexibility is worth the minor performance cost.
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