On Wed, 17 Oct 2012, Peter Ross <Peter.Ross@bogen.in-berlin.de> wrote:
http://www.freebsd.org/doc/en/books/arch-handbook/driverbasics-block.html
"Block Devices (Are Gone)" ;-)
That sounds bogus to me. Despite the lack of the BSD split device model we manage to not lose data on Linux. I think that they are advocating a BSD design flaw as a feature.
Anyway, would a zvol be significantly better than a file in the zfs as I do now?
It would probably be much the same in that they are both strings of bytes managed by the same ZFS code. Of course a file has mtime and atime fields while a block device probably doesn't.
I could imagine using LVM on Dom0 and giving partitions to the DomUs and running ZFS inside.
That means you lose the contiguous write feature of ZFS which is essential to good performance. Ext3/4 on LVM volumes gives somewhat contiguous reads where possible, ZFS when it owns the disks gives contiguous writes, but ZFS on multiple LVM volumes gives neither.
Agreed. Easy administration vs. performance.
Multiple ZFS instances is not "easy administration" IMHO. ZFS is a bit of a pain to setup, once it's going it makes some things easier, but it's not as easy as other filesystems. It's not something you do mkfs ...; mount ...
As ZFS owns the devices and the mount points it's surely not going to be easy to have multiple snapshots of a ZFS filesystem active at once. It would probably be like trying to take a snapshot of a PV that's used for LVM - something that can theoretically be usable if you take the snapshot to another system but otherwise will be a massive PITA and probably cause data loss.
I don't understand what you mean here.
With a filesystem like Ext3 you can umount it from a DomU, shutdown the DomU, and then mount it in the Dom0. It's no big deal at all. Ext3/4 with mount by UUID gets a little more complex as snapshots can result in mounting the wrong one, so you just don't use UUID mounting with LVM and similar things (LVM gives you a persistent name anyway). With something heavy like LVM and ZFS (which has similar functionality to LVM in some ways) you can't just freely mount snapshots etc. You need to scan for the devices and then the kernel keeps it's own list mapping names to devices. So if you have a second snapshot of the same device it's going to go badly wrong. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/