Neil Brown wrote:
On Tuesday October 23, dledford@xxxxxxxxxx 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
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