On Wed, 3 Aug 2016, Kent Overstreet wrote:

On Fri, May 27, 2016 at 07:45:32PM -0700, Eric Wheeler wrote:

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

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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).

I can test that once its working.  Would it use the same bcachefs tools 
for formatting superblocks?

Relatedly, can you point out the best place to abstract cachemeta-v1 vs. 
cachemeta-v2 for simultaneous use?  Could it be just a bunch of function 
pointers in the cachedev struct and assignment during initialization for 
v1/v2?  Have the call arguments changed? What functions would need 
abstractions (the smallest v1/v2 intersection)?

You mean compile a kernel that supports both old and new on disk format?

Realistically the only way that's going to happen is to completely fork the
source code, ext2/3/4 style.
Although that's going to have to happen eventually.

Sure, that makes sense.  At what point would you want to do that rename so 
bcache-dev can be pulled into the kernel tree?
	

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Can bcachefs provide /dev/bcacheN devices without loop.ko?  

If so, are these simply filesystem objects (files)?

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).

Thats a neat implementation.  Would creating a dirent for such an inode 
expose the block device with the same size and content (and ordering) if 
if the inode were compatable?  Would the blockdev be block-size aligned 
versus the file or might the file have an alignment requirement?

What we'd want to do is add an ioctl or something to take a fs inode (a normal
file, that already has a dirent) and create at runtime a block device for it.

You had mentioned changing on-disk format related to this and NFS support.  
Is that coming along too?

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I'm particularly excited about this as a precursor to snapshot support, 
especially if udev could help produce something like this:

  /dev/disk/by-path/bcache-mydiskfile -> /dev/bcacheN
  /dev/disk/by-path/bcache-mydisksnap -> /dev/bcacheN+1

Not sure what you mean by precursor - that would still require essentially the
entire snapshots implementation. But yes, once we have snapshots we could do
that too.

Precursor, as in, export an arbitrary file as a blockdev even if snapshots 
aren't ready yet.  I can start testing in our testbed once files can be 
exported as blocks, whether or not they support snapshots.

Other questions:

Is FIEMAP supported so uncached fils can be read in disk-linear order?  
Hmm, I wonder, what does FIEMAP even mean when the file spreads across 
multiple disks?  Maybe it doesn't apply here.  Really what I'm looking for 
is a way to list which blocks have changed between two snapshots for easy 
incremental backups (eg, `btrfs send`).


I'm excited about checksum support.  If an SSD bitflips, will it fail the 
whole disk, or just report an error and attempt to re-read from another 
volume?  

Right now btrfs/zfs is the only viable checksum filesystem with recovery, 
and there aren't any viable blockdevice checksumming implementations 
(dm-csum didn't take off and the PoC academic example splicing into md 
raid isn't really ready either).


--
Eric Wheeler