Re: Time to deprecate old RAID formats?

[Jeff Garzik <jeff@garzik.org>]

Neil Brown wrote:
> On Tuesday October 23, dledford@redhat.com wrote:
> As for where the metadata "should" be placed, it is interesting to
> observe that the SNIA's "DDFv1.2" puts it at the end of the device.
> And as DDF is an industry standard sponsored by multiple companies it
> must be ......
> Sorry.  I had intended to say "correct", but when it came to it, my
> fingers refused to type that word in that context.
> 
> DDF is in a somewhat different situation though.  It assumes that the
> components are whole devices, and that the controller has exclusive
> access - there is no way another controller could interpret the
> devices differently before the DDF controller has a chance.

<grin> agreed.


> DDF is also interesting in that it uses 512 byte alignment for
> metadata.  The 'anchor' block is in the last sector of the device.
> This contrasts with current md metadata which is all 4K aligned.
> Given that the drive manufacturers seem to be telling us that "4096 is
> the new 512", I think 4K alignment was a good idea.
> It could be that DDF actually specifies the anchor to reside in the
> last "block" rather than the last "sector", and it could be that the
> spec allows for block size to be device specific - I'd have to hunt
> through the spec again to be sure.

Its a bit of a mess.

Yes, with 1K and 4K sector devices starting to appear, as long as the 
underlying partitioning gets the initial partition alignment correct, 
this /should/ continue functioning as normal.

If for whatever reason you wind up with an odd-aligned 1K sector device 
and your data winds up aligned to even numbered [hard] sectors, 
performance will definitely suffer.

Mostly this is out of MD's hands, and up to the sysadmin and 
partitioning tools to get hard-sector alignment right.


> For the record, I have no intention of deprecating any of the metadata
> formats, not even 0.90.

strongly agreed


> It is conceivable that I could change the default, though that would
> require a decision as to what the new default would be.  I think it
> would have to be 1.0 or it would cause too much confusion.

A newer default would be nice.

	Jeff