Reducing ZFS fragmentation by copying to larger disk

[Bob Friesenhahn]

On Tue, 16 Jan 2018, Mark Martinec wrote:

> Having a ZFS filesystem with two 4 TB disks in a mirror that
> is 85 % full and apparently pretty much fragmented (scrubs
> or a resilver takes excessively long time), I intend to move
> its contents to a new pair of disks twice the size.
>
> My question is what method to use so that in the end the copy will
> be less fragmented:
>
> 1. attach new disks to a ZFS mirror and let them resilver, then
>   remove the old disks (Would the copied content be any less
>   fragmented, or perhaps would the added free space just
>   relieve some of the problem?)

I don't think this would help with fragmentation at all.

> 2. use zfs send / receive to make a copy
>   (is this any better than method #1 ?)

This will work well as long as there is enough space to send to.  It 
will even work within the same pool if there is enough free space.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/