DragonflyBSD GEOM? (Re: Is it time to dump disklabel and use GPT instead?)
Adam Vande More
amvandemore at gmail.com
Wed Jul 28 14:01:37 PDT 2010
On Wed, Jul 28, 2010 at 3:31 PM, Matthew Dillon <dillon at apollo.backplane.com> wrote:
Urm. What's the point of a mirror if you need a third journaling
device which itself can fail or glitch? Do you mirror the journal too?
I have serious problems with this concept, and also with the complexity
and the number of moving parts involved.There is no 2 or 3 journal. There is one journal that is lives on top of the mirrored device. A certain part of the device(defaults to 1G although that's too small for a lot of loads). A gjournal consisted of two parts, the journal itself and block device it's journaling. You can embed the journal in the device or place it on different block device. This is used for UFS file systems and is safe to mount async. I some changes were done to fsck to allow it work.
GEOM is flexible enough allow you to have a 3 journaled setup like you are talking about, but that would be silly. From an administration standpoint, managing gjournal on top of a gmirror is dead simple. Create(with a large enough journal for your io usage), and either put the support in the kernel or load it from loader.conf.
And what about the massive assumptions being made regarding the
survival of on-device write caches, write bandwidth constrictions,
the lack of integration with filesystem-derived cache flushes, the
duplication of effort when used with filesystems which have their
own UNDO/REDO journals? Is there any device stall management?
I'll be frank, the FreeBSD stuff is a non-starter for me. If we
tried to run something like gmirror and/or gjournal with HAMMER the
performance would be abysmal.Well as you say I don't think gjournal would apply to HammerFS, just like it doesn't apply to ZFS on FreeBSD, there is not much point if I understand correctly how Hammer works.
It's not just that combination though. HammerFS could be used on other modules like gvinum, gconcat, etc, etc.I'm interested in using Dragonfly for some embedded system, but having more GPL stuff is essentially a non-starter for me as well. I really like HammerFS and would have preferred to see that in FreeBSD over ZFS.
-- Adam Vande More
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