On Wed, 25 May 2016, Kent Overstreet wrote:

On Wed, May 25, 2016 at 02:47:29PM -0700, Eric Wheeler wrote:

quoted

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).

quoted

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

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