Re: [LSF/MM TOPIC] Storage: SMR drives

[Theodore Ts'o <tytso@mit.edu>]

On Thu, Jan 16, 2014 at 02:54:29PM -0200, Carlos Maiolino wrote:
> 
> I'm not sure if the decision regarding which solution to use (restricted or
> host-aware) was already made (I didn't find any doc specifying it, but in
> either case, having these values properly exported and I'm not sure Ted, if you
> had the idea to describe above all geometry information we'd like to have
> exported, but if you did, one thing I believe to be very important to us is to
> have a way to retrieve the specific location of the random write zones, so
> filesystems can take advantage of this for some specific metadata workloads.

At least initially, it will definitely be host-aware.  Even if we can
make all of our metadata be fully SMR-friendly, it's not going to deal
with random read/write updates.  So it's likely that for ext4, the
first step is to make the metadata be SMR-friendly.  The second step
will be to tweak block allocation to be SMR-friendly.  Only then would
we try to support random writes where in a SMR-friendly way --- and
it's not clear to me it makes sense for us to take things that far,
and that would be a prereq before we could take ext4 to support
restricted mode (aka host-managed SMR).

Part of what we will need to support file systems will indeed be a way
to export geometry information to userspace.

As I noted, I'm also interested in exporting support for the ZBC
commands to userspace, since there may be some use cases where the
application will manage the SMR zones, either using a raw block
device, or using ext4 to manage the large files which are aligned with
SMR zones.

Cheers,

						- Ted