Re: [PATCH RFC] fs: New zonefs file system

[Damien Le Moal <Damien.LeMoal@wdc.com>]

Jeff,

On 2019/07/18 23:11, Jeff Moyer wrote:
> Hi, Damien,
> 
> Did you consider creating a shared library?  I bet that would also
> ease application adoption for the use cases you're interested in, and
> would have similar performance.
> 
> -Jeff

Yes, it would, but to a lesser extent since system calls would need to be
replaced with library calls. Earlier work on LevelDB by Ting used the library
approach with libzbc, not quite a "libzonefs" but close enough. Working with
LevelDB code gave me the idea for zonefs. Compared to a library, the added
benefits are that specific language bindings are not a problem and further
simplify the code changes needed to support zoned block devices. In the case of
LevelDB for instance, C++ is used and file accesses are using streams, which
makes using a library a little difficult, and necessitates more changes just for
the internal application API itself. The needed changes spread beyond the device
access API.

This is I think the main advantage of this simple in-kernel FS over a library:
the developer can focus on zone block device specific needs (write sequential
pattern and garbage collection) and forget about the device access parts as the
standard system calls API can be used.

Another approach I considered is using FUSE, but went for a regular (albeit
simple) in-kernel approach due to performance concerns. While any difference in
performance for SMR HDDs would probably not be noticeable, performance would
likely be lower for upcoming NVMe zonenamespace devices compared to the
in-kernel approach.

But granted, most of the arguments I can put forward for an in-kernel FS
solution vs a user shared library solution are mostly subjective. I think though
that having support directly provided by the kernel brings zoned block devices
into the "mainstream storage options" rather than having them perceived as
fringe solutions that need additional libraries to work correctly. Zoned block
devices are not going away and may in fact become more mainstream as
implementing higher capacities more and more depends on the sequential write
interface.

Best regards.

-- 
Damien Le Moal
Western Digital Research