HAST + ZFS + NFS + CARP

[Ben RUBSON]

> 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 ?
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