ZFS mirror install /mnt is empty

[Trond Endrestøl]

On Mon, 13 May 2013 08:40-0400, Paul Kraus wrote:

> On May 13, 2013, at 1:58 AM, Trond Endrestøl <Trond.Endrestol at fagskolen.gjovik.no> wrote:
> 
> > Due to advances in hard drive technology, for the worse I'm afraid, 
> > i.e. 4K disk blocks, I wouldn't bother enabling compression on any ZFS 
> > file systems. I might change my blog posts to reflect this stop gap.
> > 
> > If you do happen to have 4K drives, you might want to check out this 
> > blog post:
> > 
> > https://ximalas.info/2012/01/11/new-server-and-first-attempt-at-running-freebsdamd64-with-zfs-for-all-storage/

>	I did look, it doesn't explain why not to enable compression on 4k sector drives.

I guess it's due to my (mis)understanding that files shorter than 4KB 
stored on 4K drives never will be subject to compression. And as you 
state below, the degree of compression depends largely on the data at 
hand.
 
> 	From discussion on the zfs-discuss lists (both the old one from OpenSolaris and the new one at Illumos) the only issue with 4K sector drives is mixing 0.5K sector and 4K sector drives. You can tunes the zpool offset to handle 4K sector drives just fine, but it is a pool wide tuning.
> 
> 	http://zfsday.com/wp-content/uploads/2012/08/Why-4k_.pdf has some 4K background, and the only mention I see of compression and 4K is that you may get less. But? you really need to test your data to see if turning compression on is beneficial with any dataset. There is noticeable computational overhead to enabling compression. If you are CPU bound, then you will get better performance with compression off. If you are limited by the I/O bandwidth to your drives, then *if* your data is highly compressible, then you will get better performance with compression on. I have managed large pools of both data that compresses well and data that does not.
> 
> 	http://wiki.illumos.org/display/illumos/ZFS+and+Advanced+Format+disks discusses the issue and presents solutions using Illumos. I could find no such examples for FreeBSD, but I'm sure some of the same techniques would work (manually setting the ashift to 12 for 4K disks).
> 
> --
> Paul Kraus
> Deputy Technical Director, LoneStarCon 3
> Sound Coordinator, Schenectady Light Opera Company

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