Re: Problem identified: WAPL/RAIDframe performance problems

[David Holland <dholland-tech%netbsd.org@localhost>]

On Sat, Dec 01, 2012 at 04:27:14PM -0500, Mouse wrote:
 > 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.

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

I don't care about the block granularity of the interface. (Unless I
suppose it's larger than the atomic write size; but that would be
weird.)

I care even less about how the media is organized internally; if it
announces that the atomic write size is 1024 bytes, it's 1024 bytes,
even if it really means that it is writing one bit each to 8192 steel
drum spindles.

Now, we have legacy code that contains additional assumptions, such as
the belief that the atomic write size is the same from device to
device, or that it can be set at newfs time rather than being a
dynamic/run-time property of the block device. And we have a lot of
code that uses DEV_BSIZE as a convenient unit of measurement and mixes
it indiscriminately with other device size properties. However, all
this stuff should be cleaned up in the long term.

It may also be necessary for lower-level code (e.g. the scsi layer) to
know more than this, but any of that can be isolated underneath the
block device interface.

-- 
David A. Holland
dholland%netbsd.org@localhost