On Sun, Jun 17, 2012 at 03:19:18PM -0500, Timothy Coalson wrote:
> 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. 


> [..] while hitachi and seagate offer 512 emulated disks

> 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 

Hitachi are native 512-byte sectors.  At least, the 5k3000 and 7k3000
are, in the 2T and 3T sizes. I haven't noticed if they have a newer
model which is 4k native.  

How long that continues to remain the case, and how long these models
continue to remain available (e.g. for replacements) is entirely
another matter.  The replacement applies even to under-warranty cases;
I know someone who recently had a 4k-only drive supplied as a warranty
replacement for a 512 native drive (not, in this case, from Hitachi).

As for performance, at least in my experience with WD disks
emulating 512-byte sectors, you *will* notice the difference; heavy
metadata updates being the most obvious impact.

The conclusion is that unless your environment is well controlled, the
time has probably come where new general-purpose pools should be made
at ashift=12, to allow future flexibility.

> 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).

SATA port-multipliers and SAS expanders are not related in any sense
of common driver support; they're similar only in general concept. 

Do not conflate them.


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