HAST + ZFS + NFS + CARP

[Julien Cigar]

On Wed, Aug 17, 2016 at 09:25:30AM +0200, InterNetX - Juergen Gotteswinter wrote:
> 
> 
> Am 11.08.2016 um 11:24 schrieb Borja Marcos:
> > 
> >> On 11 Aug 2016, at 11:10, Julien Cigar <julien at perdition.city> wrote:
> >>
> >> As I said in a previous post I tested the zfs send/receive approach (with
> >> zrep) and it works (more or less) perfectly.. so I concur in all what you
> >> said, especially about off-site replicate and synchronous replication.
> >>
> >> Out of curiosity I'm also testing a ZFS + iSCSI + CARP at the moment, 
> >> I'm in the early tests, haven't done any heavy writes yet, but ATM it 
> >> works as expected, I havent' managed to corrupt the zpool.
> > 
> > I must be too old school, but I don’t quite like the idea of using an essentially unreliable transport
> > (Ethernet) for low-level filesystem operations.
> > 
> > In case something went wrong, that approach could risk corrupting a pool. Although, frankly,
> > ZFS is extremely resilient. One of mine even survived a SAS HBA problem that caused some
> > silent corruption.
> 
> try dual split import :D i mean, zpool -f import on 2 machines hooked up
> to the same disk chassis.

Yes this is the first thing on the list to avoid .. :)

I'm still busy to test the whole setup here, including the 
MASTER -> BACKUP failover script (CARP), but I think you can prevent
that thanks to:

- As long as ctld is running on the BACKUP the disks are locked 
and you can't import the pool (even with -f) for ex (filer2 is the
BACKUP):
https://gist.github.com/silenius/f9536e081d473ba4fddd50f59c56b58f

- The shared pool should not be mounted at boot, and you should ensure
that the failover script is not executed during boot time too: this is
to handle the case wherein both machines turn off and/or re-ignite at
the same time. Indeed, the CARP interface can "flip" it's status if both
machines are powered on at the same time, for ex:
https://gist.github.com/silenius/344c3e998a1889f988fdfc3ceba57aaf and
you will have a split-brain scenario

- Sometimes you'll need to reboot the MASTER for some $reasons
(freebsd-update, etc) and the MASTER -> BACKUP switch should not
happen, this can be handled with a trigger file or something like that

- I've still have to check if the order is OK, but I think that as long
as you shutdown the replication interface and that you adapt the
advskew (including the config file) of the CARP interface before the 
zpool import -f in the failover script you can be relatively confident 
that nothing will be written on the iSCSI targets

- A zpool scrub should be run at regular intervals

This is my MASTER -> BACKUP CARP script ATM
https://gist.github.com/silenius/7f6ee8030eb6b923affb655a259bfef7

Julien

> 
> kaboom, really ugly kaboom. thats what is very likely to happen sooner
> or later especially when it comes to homegrown automatism solutions.
> even the commercial parts where much more time/work goes into such
> solutions fail in a regular manner
> 
> > 
> > The advantage of ZFS send/receive of datasets is, however, that you can consider it
> > essentially atomic. A transport corruption should not cause trouble (apart from a failed
> > "zfs receive") and with snapshot retention you can even roll back. You can’t roll back
> > zpool replications :)
> > 
> > ZFS receive does a lot of sanity checks as well. As long as your zfs receive doesn’t involve a rollback
> > to the latest snapshot, it won’t destroy anything by mistake. Just make sure that your replica datasets
> > aren’t mounted and zfs receive won’t complain.
> > 
> > 
> > Cheers,
> > 
> > 
> > 
> > 
> > Borja.
> > 
> > 
> > 
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> > 
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-- 
Julien Cigar
Belgian Biodiversity Platform (http://www.biodiversity.be)
PGP fingerprint: EEF9 F697 4B68 D275 7B11  6A25 B2BB 3710 A204 23C0
No trees were killed in the creation of this message.
However, many electrons were terribly inconvenienced.
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