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