Ah, yes -- now we're talking about a meaningful way in which ZFS can be improved! (But then, you know that.) Metaslab fragmentation is a very real issue -- and

Ah, yes -- now we're talking about a meaningful way in which ZFS can be improved! (But then, you know that.) Metaslab fragmentation is a very real issue -- and (as you point out) when pools are driven up in terms of utilization, that fragmentation can become acute. (Uday Vallamsetty at Delphix has an excellent blog entry that explores this visually and quantitatively.[1]) In terms of fixing it: ZFS co-inventor Matt Ahrens did extensive prototyping work on block pointer rewrite, but the results were mixed[2] -- and it was a casualty of the Oracle acquisition regardless. I don't know if the answer is rewriting blocks or behaving better when the system has become fragmented (or both), but this is surely a domain in which ZFS can be improved. I would encourage anyone interested in taking a swing at this problem to engage with Uday and others in the community -- and to attend the next ZFS Day[3]. [1] http://blog.delphix.com/uday/2013/02/19/78/ http://blog.delphix.com/uday/2013/02/19/78/ [2] http://permalink.gmane.org/gmane.os.illumos.devel/5203 http://permalink.gmane.org/gmane.os.illumos.devel/5203 [3] http://zfsday.com/zfsday/ http://zfsday.com/zfsday/