Re: Problem identified: WAPL/RAIDframe performance problems

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>> Neither.  The sector size claimed to the host should equal both the
>> sector size on the media and the granularity of the interface.
> As a consumer of block devices, I don't care about either of these
> things.  What I care about is the largest size "sector" that will (in
> the ordinary course of things anyway) be written atomically.

Then those are 512-byte-sector drives as far as you're concerned; you
can ignore the 4K reality.  At least, absent bugs in the drives, but
that's always a valid caveat.

This is because the RMW cycle that goes on internally for sub-4K writes
is invisible: a 512-byte write always either has completed in full or
has not yet started at all as far as all other interactions with the
drive goes.  That is, such writes (and reads) are atomic.

It's a coherent point of view.  But it's one I don't share; I care more
about performance than that.  This is why I care about visibility into
internal organization.

> I might also care about larger sizes that the drive considers
> significant for alignment purposes; but probably not very much.

That depends on whether you care about performance.

> I don't care about the block granularity of the interface.

Don't you pretty much have to care about it, since that's the unit in
which data addresses are presented to its interface?  Or is that
something you believe should be hidden by...something else?  (It's not
clear to me exactly what the `you' that doesn't care about interface
granularity includes - hardware driver authors? filesystem authors?
midlayer (eg scsipi) authors?)

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