geom_raid5 inclusion in HEAD?

Oliver Fromme olli at lurza.secnetix.de
Thu Nov 15 07:14:33 PST 2007


Arne Wörner wrote:
 > Oliver Fromme wrote:
 > > Just a small question:  I noticed that the new gvinum
 > > raid5 implementation (in P4) allows adding disks to an
 > > existing RAID5, even while it is running.  Does geom_raid5
 > > support that, too?  (ZFS doesn't, unfortunately.)
 > 
 > Nope... graid5 doesnt do such things... I found no way, that could do it
 > without hurting the disks too much (I was afraid, that a power failure could
 > destroy the necessary knowledge about the size of the new-config-area; and I
 > didnt know how to do the beginning: it seemed like the first few blocks need a
 > special treatment, because there the new-config-area and the old-config-area
 > overlap)...

OK.  I don't know the inner workings of geom_raid5, so I
can't tell how difficult it would be to implement there.

Here's a little description and some ASCII graphics that
explin how growing RAID5 was implemented in the new
gvinum:

http://lists.freebsd.org/pipermail/p4-projects/2007-July/020082.html

 > But Veronica is developing a tool, that can do it in offline mode... With
 > service interruption...
 > 
 > But growfs induces a service interruption anyway and it is buggy, if u do
 > not zero the new area... Veronica filed a bug report about this...

Hm.  I used growfs only once, and it worked fine.  Was
there a regression introduced at some point?  It should
certainly be fixed, because growfs seem to be very
useful.

About service interruption:  growfs only takes a few
seconds, which might be acceptable in most cases.
But taking a whole RAID5 down to add disks and then
rebuilding it takes a _lot_ longer.  Therefore I think
the feature to add disks to a live RAID5 would be very
valuable.

 > Nowadays it is common practice to have 2 ot more hosts, that can substitute
 > each other (hot-standby or how they call it today), so that it doesnt matter,
 > if a box is damaged or in maintenance mode or... isnt it?

It depends.  Building a fail-over cluster with FreeBSD
is not trivial if you need a synchronized, consistent and
reliable file system on all of the nodes.

Of course you can use third-party black boxes such as
a cluster of NetApp Filers or whatever.  That would work
(I've put such setups into production myself), but it
costs a non-negligible amount of money, and it's
certainly not suitable for everyone.

YMMV, of course.

Best regards
   Oliver

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