Re: [PATCH v6 0/5] Initial support for multi-actuator HDDs

On 2021/08/28 1:43, Christoph Hellwig wrote:

On Fri, Aug 27, 2021 at 02:28:58PM +0000, Tim Walker wrote:

quoted

There is nothing in the spec that requires the ranges to be contiguous
or non-overlapping.

Yikes, that is a pretty stupid standard.  Almost as bad as allowing
non-uniform sized non-power of two sized zones :)

quoted

It's easy to imagine a HDD architecture that allows multiple heads to access the same sectors on the disk. It's also easy to imagine a workload scenario where parallel access to the same disk could be useful. (Think of a typical storage design that sequentially writes new user data gradually filling the disk, while simultaneously supporting random user reads over the written data.)

But for those drivers you do not actually need this scheme at all.

Agree.

Storage devices that support higher concurrency are bog standard with
SSDs and if you want to go back storage arrays.  The only interesting
case is when these ranges are separate so that the access can be carved
up based on the boundary.  Now I don't want to give people ideas with
overlapping but not identical, which would be just horrible.

Agree too. And looking at my patch again, the function disk_check_iaranges() in
patch 1 only checks that the overall sector range of all access ranges is form 0
to capacity - 1, but it does not check for holes nor overlap. I need to change
that and ignore any disk that reports overlapping ranges or ranges with holes in
the LBA space. Holes would be horrible and if we have overlap, then the drive
can optimize by itself. Will resend a V7 with corrections for that.


-- 
Damien Le Moal
Western Digital Research