HAST + ZFS + NFS + CARP

[Ben RUBSON]

> On 04 Jul 2016, at 00:24, Jordan Hubbard <jkh at ixsystems.com> wrote:
> 
>> On Jul 3, 2016, at 2:47 PM, Julien Cigar <julien at perdition.city> wrote:
>> 
>> I guess that ZFS will split the read requests accross all devices in
>> order to maximize performance... which could lead to contrary to what is
>> expecpted when iSCSI disks are involved, no?
>> Is there some sysctl params which could prevent this unexpected
>> behavior?
> 
> Nope.  You will suffer the performance implications of layering a filesystem that expects “rotating media or SSDs” (with the innate ability to parallelize multiple requests in a way that ADD performance) on top of a system which is now serializing the requests across an internet connection to another software layer which may offer no performance benefits to having multiple LUNs at all.   You can try iSCSI-specific tricks like MPIO to try and increase performance, but ZFS itself is just going to treat everything it sees as “a disk” and so physical concepts like mirrors or multiple vdevs for performance won’t translate across.
> 
> Example question: What’s the point of writing multiple copies of data across virtual disks in a mirror configuration if the underlying storage for the virtual disks is already redundant and the I/Os to it serialize?
> Example Answer: There is no point.  In fact, it’s a pessimization to do so.

Of course Jordan, in this topic, we (well at least me :) make the following assumption :
one iSCSI target/disk = one real physical disk (a SAS disk, a SSD disk...), from a server having its own JBOD, no RAID adapter or whatever, just what ZFS likes !

> This is not a lot different than running ZFS on top of RAID controllers that turn N physical disks into 1 or more virtual disks.  You have to make entirely different performance decisions based on such scenarios and that’s just the way it is, which is also why we don’t recommend doing that.

Of course you loose all ZFS benefits if you only mirror 2 "disks", a big one from storage array A, the same from storage array B.
No interest.