[linux-lvm] Configuring extent size

[hansbkk at gmail.com]

There is a lot of apparently authoritative but conflicting information
on this topic out on the interwebs. I suspect the problem is that
changes from LVM1 to LVM2 weren't explicitly highlighted.

For example, I believe this is no longer true, but it still figures
prominently in returns from google:

> a 4 MB extent size imposes a logical volume size limitation of 256 Gigabytes

I believe the below is now true, and that this is the reason the above
is no longer relevant:

> with LVM2, there's no limit on the maximum numbers of extents per PV/LV.

IOW, the FAQ's statement:

> For 32-bit CPUs on 2.6 kernels, the maximum LV size is 16TB.

is a hard upper limit, and is not related to extent size - please confirm.


And is there a maximum size for PV or VG and would this be related to
extent size?


Also, the man page state that the *default* extent size (if none is
specified) is 4MB, but the HowTo states it's 32MB. Which is true?

Or does this vary by vgcreate version, or per-distro configuration?


Finally

> having a large number of extents will slow down the tools

leads me to:

Would there be any negative impact of a very large extent size?

I'm thinking of using 1GB (4096*256=1048576). This would make extents
human-friendly when used directly as units in allocating space. Even
10 GB would only be 2% of the smallest drive I'm using, so wouldn't
hamper flexibility, but IMO this seems a bit "out there".

Any and all feedback welcome.