[linux-lvm] Extend VG - Expand LUN vs New Disk

[linux-lvm] Extend VG - Expand LUN vs New Disk

[Jorge Fábregas]

Hi everyone,

Are there any major differences between expanding an existing LUN
(already striped & mirrored on storage-array) and use pvresize &
"lvextend -r ..." vs adding a new disk to your VG when you need to
expand an existing filesystem?

At work we do the latter and almost everyone believes expanding an
existing LUN is not a safe operation.  Is there really a preferred
method? Or is it just a matter of personal preference?

Thanks!
Jorge

Re: [linux-lvm] Extend VG - Expand LUN vs New Disk

[Bryn M. Reeves]

On Mon, Dec 21, 2015 at 08:30:29AM -0400, Jorge F�bregas wrote:
> Are there any major differences between expanding an existing LUN
> (already striped & mirrored on storage-array) and use pvresize &
> "lvextend -r ..." vs adding a new disk to your VG when you need to
> expand an existing filesystem?
 
Expanding an existing LUN may involve resizing partitions - that is
normally the step that makes this the more complicated route since
depending on system configuration and use it may not be easy to resize
the partition without downtime (the Linux kernel has for some time
enforced restrictions on resizing in-use partitions - device-mapper
devices and whole disk devices do not share this limitation).

> At work we do the latter and almost everyone believes expanding an
> existing LUN is not a safe operation.  Is there really a preferred
> method? Or is it just a matter of personal preference?

It's safe enough - it's just that the kernel generally won't let you
do it while the existing partition is active and in use.

Expanding an existing partition may be seen as the "cleaner" option,
since it avoids creating additional PVs and associated labels and
metadata areas, but unless you are working with very large numbers of
devices it makes little difference to most operations.

Gluing lots of devices together is one of the main reasons for using a
volume manager and for typical uses it adds no appreciable overhead
or additional complexity.

Regards,
Bryn.

Re: [linux-lvm] Extend VG - Expand LUN vs New Disk

[Jorge Fábregas]

On 12/21/2015 08:50 AM, Bryn M. Reeves wrote:
> Expanding an existing LUN may involve resizing partitions - that is
> normally the step that makes this the more complicated route since
> depending on system configuration and use it may not be easy to resize
> the partition without downtime (the Linux kernel has for some time
> enforced restrictions on resizing in-use partitions - device-mapper
> devices and whole disk devices do not share this limitation).

We use whole disks all the time :)

> It's safe enough - it's just that the kernel generally won't let you
> do it while the existing partition is active and in use.

I'm glad to know that:  one more reason to use whole disks.  I know Red
Hat favors partitions for PVs (as mentioned in various docs) but I've
never been convinced by the arguments for it.


> Expanding an existing partition may be seen as the "cleaner" option,
> since it avoids creating additional PVs and associated labels and
> metadata areas, but unless you are working with very large numbers of
> devices it makes little difference to most operations.

Excellent point.

> Gluing lots of devices together is one of the main reasons for using a
> volume manager and for typical uses it adds no appreciable overhead
> or additional complexity.

Thanks Bryn for your wonderful feedback.  I appreciate it!

All the best,
Jorge