Urgently need some help solving lost space due to snapshots during receive op

[PK1048]

On Aug 10, 2015, at 4:42, Willem Jan Withagen <wjw at digiware.nl> wrote:

> Last number I remember as sensible maximum fill level is around 70%.
> If you go over it, your system is going to be busy with ZFS bookkeeping
> instead of data storage.

Fortunately or not, that number is _very_ dependent on your specific data and how it is loaded. If it is static data, loaded once and not modified, you can run a much higher % Used, for lots of changes on an ongoing basis you need much more free space. The issue is that since ZFS is Copy on Write, every write (even a modification to an existing file or disk block) needs to find space. ZFS tries to reduce fragmentation. As the zpool gets more and more full it takes longer and longer to find the best possible location to put the new write. It also becomes harder and harder to find contiguous blocks to put the new write into. The existence of snapshots makes this problem even worse, as previously written data is not cleared until the snapshot is destroyed.