> So I can either exchange the disks one by one with autoexpand, use 2-4 TB
> disks and be happy. This was my original approach. However I am totally
> unclear about the 512b vs 4Kb issue. What sata disk could I use that is big
> enough and still uses 512b? I know about the discussion about the upgrade
> from a 512b based pool to a 4 KB pool but I fail to see a conclusion. Will
> the autoexpand mechanism upgrade ashift? And what disks do not lie? Is the
> performance impact significant?

Replacing devices will not change the ashift, it is set permanently
when a vdev is created, and zpool will refuse to replace a device in
an ashift=9 vdev with a device that it would use ashift=12 on.  Large
Western Digital disks tend to say they have 4k sectors, and hence
cannot be used to replace your current disks, while hitachi and
seagate offer 512 emulated disks, which should allow you to replace
your current disks without needing to copy the contents of the pool to
a new one.  If you don't have serious performance requirements, you
may not notice the impact of emulated 512 sectors (especially since
zfs buffers async writes into transaction groups).  I did some
rudimentary testing on a large pool of hitachi 3TB 512 emulated disks
with ashift=9 vs ashift=12 with bonnie, and it didn't seem to matter a
whole lot (though its possibly relevant tests were large writes, which
have little penalty, and character at a time, which was bottlenecked
by the cpu since the test was single threaded, so it didn't test the
worst case).  The worst case for 512 emulated sectors on zfs is
probably small (4KB or so) synchronous writes (which if they mattered
to you, you would probably have a separate log device, in which case
the data disk write penalty may not matter).

> So I started to think about option 2. That would be using an external JOBD
> chassis (4-8 disks) and eSATA. But I would either need a JBOD with 4-8 eSATA
> connectors (which I am yet to find) or use a JBOD with a "good" expander. I
> see several cheap sata to esata jbod chassis making use of "port
> multiplier". Is this referring to a expander backplane and will work with
> oi, LSI and mpt or mpt_sas?

I'm wondering, based on the comment about routing 4 eSATA cables, what
kind of options your NAS case has, if your LSI controller has SFF-8087
connectors (or possibly even if it doesn't), you might be able to use
an adapter to the SFF-8088 external 4 lane SAS connector, which may
increase your options.  It seems that support for SATA port multiplier
is not mandatory in a controller, so you will want to check with LSI
before trying it (I would hope they support it on SAS controllers,
since I think it is a vastly simplified version of SAS expanders).

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