HAST + ZFS + NFS + CARP

[Julien Cigar]

On Thu, Jun 30, 2016 at 05:28:41PM +0200, Ben RUBSON wrote:
> 
> > On 30 Jun 2016, at 17:14, InterNetX - Juergen Gotteswinter <jg at internetx.com> wrote:
> > 
> > 
> > 
> > Am 30.06.2016 um 16:45 schrieb Julien Cigar:
> >> Hello,
> >> 
> >> I'm always in the process of setting a redundant low-cost storage for 
> >> our (small, ~30 people) team here.
> >> 
> >> I read quite a lot of articles/documentations/etc and I plan to use HAST
> >> with ZFS for the storage, CARP for the failover and the "good old NFS"
> >> to mount the shares on the clients.
> >> 
> >> The hardware is 2xHP Proliant DL20 boxes with 2 dedicated disks for the
> >> shared storage.
> >> 
> >> Assuming the following configuration:
> >> - MASTER is the active node and BACKUP is the standby node.
> >> - two disks in each machine: ada0 and ada1.
> >> - two interfaces in each machine: em0 and em1
> >> - em0 is the primary interface (with CARP setup)
> >> - em1 is dedicated to the HAST traffic (crossover cable)
> >> - FreeBSD is properly installed in each machine.
> >> - a HAST resource "disk0" for ada0p2.
> >> - a HAST resource "disk1" for ada1p2.
> >> - a zpool create zhast mirror /dev/hast/disk0 /dev/hast/disk1 is created
> >>  on MASTER
> >> 
> >> A couple of questions I am still wondering:
> >> - If a disk dies on the MASTER I guess that zpool will not see it and
> >>  will transparently use the one on BACKUP through the HAST ressource..
> > 
> > thats right, as long as writes on $anything have been successful hast is
> > happy and wont start whining
> > 
> >>  is it a problem? 
> > 
> > imho yes, at least from management view
> > 
> >> could this lead to some corruption?
> > 
> > probably, i never heard about anyone who uses that for long time in
> > production
> > 
> > At this stage the
> >>  common sense would be to replace the disk quickly, but imagine the
> >>  worst case scenario where ada1 on MASTER dies, zpool will not see it 
> >>  and will transparently use the one from the BACKUP node (through the 
> >>  "disk1" HAST ressource), later ada0 on MASTER dies, zpool will not 
> >>  see it and will transparently use the one from the BACKUP node 
> >>  (through the "disk0" HAST ressource). At this point on MASTER the two 
> >>  disks are broken but the pool is still considered healthy ... What if 
> >>  after that we unplug the em0 network cable on BACKUP? Storage is
> >>  down..
> >> - Under heavy I/O the MASTER box suddently dies (for some reasons), 
> >>  thanks to CARP the BACKUP node will switch from standy -> active and 
> >>  execute the failover script which does some "hastctl role primary" for
> >>  the ressources and a zpool import. I wondered if there are any
> >>  situations where the pool couldn't be imported (= data corruption)?
> >>  For example what if the pool hasn't been exported on the MASTER before
> >>  it dies?
> >> - Is it a problem if the NFS daemons are started at boot on the standby
> >>  node, or should they only be started in the failover script? What
> >>  about stale files and active connections on the clients?
> > 
> > sometimes stale mounts recover, sometimes not, sometimes clients need
> > even reboots
> > 
> >> - A catastrophic power failure occur and MASTER and BACKUP are suddently
> >>  powered down. Later the power returns, is it possible that some
> >>  problem occur (split-brain scenario ?) regarding the order in which the
> > 
> > sure, you need an exact procedure to recover
> > 
> >>  two machines boot up?
> > 
> > best practice should be to keep everything down after boot
> > 
> >> - Other things I have not thought?
> >> 
> > 
> > 
> > 
> >> Thanks!
> >> Julien
> >> 
> > 
> > 
> > imho:
> > 
> > leave hast where it is, go for zfs replication. will save your butt,
> > sooner or later if you avoid this fragile combination
> 
> I was also replying, and finishing by this :
> Why don't you set your slave as an iSCSI target and simply do ZFS mirroring ?

Yes that's another option, so a zpool with two mirrors (local + 
exported iSCSI) ?

> ZFS would then know as soon as a disk is failing.
> And if the master fails, you only have to import (-f certainly, in case of a master power failure) on the slave.
> 
> Ben
> 
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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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