Re: [LSF/MM/BPF BoF] BoF for Zoned Storage

[Adam Manzanares <a.manzanares@samsung.com>]

On Thu, Mar 03, 2022 at 07:51:36PM +0000, Matias Bjørling wrote:
> > Sounds like you voluntered to teach zoned storage use 101. Can you teach me
> > how to calculate an LBA offset given a zone number when zone capacity is not
> > equal to zone size?
> 
> zonesize_pow = x; // e.g., x = 32 if 2GiB Zone size /w 512B block size
> zone_id = y; // valid zone id
> 
> struct blk_zone zone = zones[zone_id]; // zones is a linear array of blk_zone structs that holds per zone information.
> 
> With that, one can do the following
> 1a) first_lba_of_zone =  zone_id << zonesize_pow;
> 1b) first_lba_of_zone = zone.start;

1b is interesting. What happens if i don't have struct blk_zone and zone size 
is not equal to zone capacity?

> 2a) next_writeable_lba = (zoneid << zonesize_pow) + zone.wp;
> 2b) next_writeable_lba = zone.start + zone.wp;

Can we modify 2b to not use zone.start?

> 3)   writeable_lbas_left = zone.len - zone.wp;
> 4)   lbas_written = zone.wp - 1;
> 
> > The second thing I would like to know is what happens when an application
> > wants to map an object that spans multiple consecutive zones. Does the
> > application have to be aware of the difference in zone capacity and zone size?
> 
> The zoned namespace command set specification does not allow variable zone size. The zone size is fixed for all zones in a namespace. Only the zone capacity has the capability to be variable. Usually, the zone capacity is fixed, I have not yet seen implementations that have variable zone capacities.
> 

IDK where variable zone size came from. I am talking about the fact that the 
zone size does not have to equal zone capacity. 

> An application that wants to place a single object across a set of zones would have to be explicitly handled by the application. E.g., as well as the application, should be aware of a zone's capacity, it should also be aware that it should reset the set of zones and not a single zone. I.e., the application must always be aware of the zones it uses.
> 
> However, an end-user application should not (in my opinion) have to deal with this. It should use helper functions from a library that provides the appropriate abstraction to the application, such that the applications don't have to care about either specific zone capacity/size, or multiple resets. This is similar to how file systems work with file system semantics. For example, a file can span multiple extents on disk, but all an application sees is the file semantics. 
> 

I don't want to go so far as to say what the end user application should and 
should not do.