Re: bcachefs: can bcachefs export block devices?

[Eric Wheeler <bcache@lists.ewheeler.net>]

On Wed, 25 May 2016, Kent Overstreet wrote:

> On Wed, May 25, 2016 at 02:47:29PM -0700, Eric Wheeler wrote:
> > 
> > I have a few bcachefs questions that pertain to its use with block 
> > devices.  
> > 
> > Does bcachefs's implementation reuse and update the existing 
> > bcache code such that the block device driver inherits the bcachefs 
> > improvements?  I understand the cache superblock changed, maybe the cached 
> > dev super too.
> 
> Yes, all of the existing functionality is still there (though some of it's
> broken at the moment because I haven't been running those tests; if you're
> interested in using bcache-dev for the old style caching (there are performance
> and robustness improvements) it wouldn't take me long to get it working again).
> 
> > Can bcachefs provide /dev/bcacheN devices without loop.ko?  
> > 
> > If so, are these simply filesystem objects (files)?
> 
> No, at least not currently - the "export a block device" code and the filesystem
> code are effectively thin wrappers around the core bcache IO path (bch_read()
> and bch_write()) - but the two different interfaces don't have anything to do
> with each other.
> 
> The way it works is the first 4096 inode numbers are owned by the block device
> interface - inodes in that range are for either cached devices or thin
> provisioned volumes. The filesystem code owns inode numbers >= 4096.
> 
> So while blockdev volumes/cached data do have inodes, they're not reachable via
> the filesystem because there will never be dirents that point to them (also,
> they use a different inode type with extra fields for the UUID/label).
> 
> However - there isn't anything preventing us from writing a bit of new code and
> hooking it up to an ioctl or sysfs interface to say "look up this path and
> create a block device for that file". The only remotely tricky bit would be
> pagecache cache coherency, but I think you get the new block device to just use
> the same address_space (pagecache cache for an inode) as the filesystem inode.
> 
> So, probably doable in ~100 lines of code or so.

Hey Kent,

I noticed that blockdev.c got an overhaul in bcache-dev recently.  Getting 
closer to blockdev export from bcachefs?

Actually it looks like you've been pretty busy on that branch in general.  

What's new?  


--
Eric Wheeler



> --
> To unsubscribe from this list: send the line "unsubscribe linux-bcache" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>