Q:

Should I really leave some space unused with LVM partition?

I am migrating to a new server where I'm going to use LVM partitioning on a RAID 1 array (2 hard drives). I have read some tutorials which advise to leave some space unused so that I can  expand the partition later, if needed.
Now, with the new hardware that is large for my uses, I could afford that, but I wonder if this makes any sense: why would I do that? Is it somehow easier, say, to expand /var into unused space later on, than to shrink main partition and expand /var?

A:

Should you... depends.  One huge reason to keep extra space around is so you can use LVM snapshots.  With no free space you cannot use them.
As for the reasoning 'so you can expand into it later'.. meh.  I'd rather size accordingly and not run into the need to expand later on.  Expanding 0 times is easier/faster than expanding 1 time.
As for being easier to simply expand vs shrink and expand, it's obviously easier.  One command to grow the logical volume, another to grow the filesystem -vs- one to shrink the filesystem, one to shrink the logical volume (of shrunken filesystem), one to grow the other logical volume, another to expand the filesystem.
If you're comfortable shrinking filesystems and logical volumes then it's not really any harder.  Just more steps.

A:

to expand /var into unused space later on, than to shrink main partition 

Expanding logical volumes and filesystems can be done while the system is running and the filesystem is mounted (for the more common filesystems).  Shrinking a filesystem means you have to unmount that filesystem (for the FS, that support a shrink).  If you are talking about your root, or some other critical filesystem, this means taking your system offline and possibly booting from some kind of rescue media to perform the resize of the filesystem and volume.
So leaving yourself free space means you can expand a filesystem, or create new ones when you need to with little or no apparent impact on normal operations.

