On Fri, Dec 3, 2010 at 1:45 PM, Josef Bacik <jo...@redhat.com> wrote: > On Wed, Dec 01, 2010 at 09:21:36AM -0500, Josef Bacik wrote: >> Hello, >> >> Various people have complained about how BTRFS deals with subvolumes >> recently, >> specifically the fact that they all have the same inode number, and there's >> no >> discrete seperation from one subvolume to another. Christoph asked that I >> lay >> out a basic design document of how we want subvolumes to work so we can hash >> everything out now, fix what is broken, and then move forward with a design >> that >> everybody is more or less happy with. I apologize in advance for how >> freaking >> long this email is going to be. I assume that most people are generally >> familiar with how BTRFS works, so I'm not going to bother explaining in great >> detail some stuff. >> >> === What are subvolumes? === >> >> They are just another tree. In BTRFS we have various b-trees to describe the >> filesystem. A few of them are filesystem wide, such as the extent tree, >> chunk >> tree, root tree etc. The tree's that hold the actual filesystem data, that >> is >> inodes and such, are kept in their own b-tree. This is how subvolumes and >> snapshots appear on disk, they are simply new b-trees with all of the file >> data >> contained within them. >> >> === What do subvolumes look like? === >> >> All the user sees are directories. They act like any other directory acts, >> with >> a few exceptions >> >> 1) You cannot hardlink between subvolumes. This is because subvolumes have >> their own inode numbers and such, think of them as seperate mounts in this >> case, >> you cannot hardlink between two mounts because the link needs to point to the >> same on disk inode, which is impossible between two different filesystems. >> The >> same is true for subvolumes, they have their own trees with their own inodes >> and >> inode numbers, so it's impossible to hardlink between them. >> >> 1a) In case it wasn't clear from above, each subvolume has their own inode >> numbers, so you can have the same inode numbers used between two different >> subvolumes, since they are two different trees. >> >> 2) Obviously you can't just rm -rf subvolumes. Because they are roots >> there's >> extra metadata to keep track of them, so you have to use one of our ioctls to >> delete subvolumes/snapshots. >> >> But permissions and everything else they are the same. >> >> There is one tricky thing. When you create a subvolume, the directory inode >> that is created in the parent subvolume has the inode number of 256. So if >> you >> have a bunch of subvolumes in the same parent subvolume, you are going to >> have a >> bunch of directories with the inode number of 256. This is so when users cd >> into a subvolume we can know its a subvolume and do all the normal voodoo to >> start looking in the subvolumes tree instead of the parent subvolumes tree. >> >> This is where things go a bit sideways. We had serious problems with NFS, >> but >> thankfully NFS gives us a bunch of hooks to get around these problems. >> CIFS/Samba do not, so we will have problems there, not to mention any other >> userspace application that looks at inode numbers. >> >> === How do we want subvolumes to work from a user perspective? === >> >> 1) Users need to be able to create their own subvolumes. The permission >> semantics will be absolutely the same as creating directories, so I don't >> think >> this is too tricky. We want this because you can only take snapshots of >> subvolumes, and so it is important that users be able to create their own >> discrete snapshottable targets. >> >> 2) Users need to be able to snapshot their subvolumes. This is basically the >> same as #1, but it bears repeating. >> >> 3) Subvolumes shouldn't need to be specifically mounted. This is also >> important, we don't want users to have to go around mounting their >> subvolumes up >> manually one-by-one. Today users just cd into subvolumes and it works, just >> like cd'ing into a directory. >> >> === Quotas === >> >> This is a huge topic in and of itself, but Christoph mentioned wanting to >> have >> an idea of what we wanted to do with it, so I'm putting it here. There are >> really 2 things here >> >> 1) Limiting the size of subvolumes. This is really easy for us, just create >> a >> subvolume and at creation time set a maximum size it can grow to and not let >> it >> go farther than that. Nice, simple and straightforward. >> >> 2) Normal quotas, via the quota tools. This just comes down to how do we >> want >> to charge users, do we want to do it per subvolume, or per filesystem. My >> vote >> is per filesystem. Obviously this will make it tricky with snapshots, but I >> think if we're just charging the diff's between the original volume and the >> snapshot to the user then that will be the easiest for people to understand, >> rather than making a snapshot all of a sudden count the users currently used >> quota * 2. >> >> === What do we do? === >> >> This is where I expect to see the most discussion. Here is what I want to do >> >> 1) Scrap the 256 inode number thing. Instead we'll just put a flag in the >> inode >> to say "Hey, I'm a subvolume" and then we can do all of the appropriate magic >> that way. This unfortunately will be an incompatible format change, but the >> sooner we get this adressed the easier it will be in the long run. Obviously >> when I say format change I mean via the incompat bits we have, so old fs's >> won't >> be broken and such. >> >> 2) Do something like NFS's referral mounts when we cd into a subvolume. Now >> we >> just do dentry trickery, but that doesn't make the boundary between >> subvolumes >> clear, so it will confuse people (and samba) when they walk into a subvolume >> and >> all of a sudden the inode numbers are the same as in the directory behind >> them. >> With doing the referral mount thing, each subvolume appears to be its own >> mount >> and that way things like NFS and samba will work properly. >> >> I feel like I'm forgetting something here, hopefully somebody will point it >> out. >> >> === Conclusion === >> >> There are definitely some wonky things with subvolumes, but I don't think >> they >> are things that cannot be fixed now. Some of these changes will require >> incompat format changes, but it's either we fix it now, or later on down the >> road when BTRFS starts getting used in production really find out how many >> things our current scheme breaks and then have to do the changes then. >> Thanks, >> > > So now that I've actually looked at everything, it looks like the semantics > are > all right for subvolumes > > 1) readdir - we return the root id in d_ino, which is unique across the fs > 2) stat - we return 256 for all subvolumes, because that is their inode number > 3) dev_t - we setup an anon super for all volumes, so they all get their own > dev_t, which is set properly for all of their children, see below > > [r...@test1244 btrfs-test]# stat . > File: `.' > Size: 20 Blocks: 8 IO Block: 4096 directory > Device: 15h/21d Inode: 256 Links: 1 > Access: (0555/dr-xr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root) > Access: 2010-12-03 15:35:41.931679393 -0500 > Modify: 2010-12-03 15:35:20.405679493 -0500 > Change: 2010-12-03 15:35:20.405679493 -0500 > > [r...@test1244 btrfs-test]# stat foo > File: `foo' > Size: 12 Blocks: 0 IO Block: 4096 directory > Device: 19h/25d Inode: 256 Links: 1 > Access: (0700/drwx------) Uid: ( 0/ root) Gid: ( 0/ root) > Access: 2010-12-03 15:35:17.501679393 -0500 > Modify: 2010-12-03 15:35:59.150680051 -0500 > Change: 2010-12-03 15:35:59.150680051 -0500 > > [r...@test1244 btrfs-test]# stat foo/foobar > File: `foo/foobar' > Size: 0 Blocks: 0 IO Block: 4096 regular empty file > Device: 19h/25d Inode: 257 Links: 1 > Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root) > Access: 2010-12-03 15:35:59.150680051 -0500 > Modify: 2010-12-03 15:35:59.150680051 -0500 > Change: 2010-12-03 15:35:59.150680051 -0500 > > So as far as the user is concerned, everything should come out right. > Obviously > we had to do the NFS trickery still because as far as VFS is concerned the > subvolumes are all on the same mount. So the question is this (and really > this > is directed at Christoph and Bruce and anybody else who may care), is this > good > enough, or do we want to have a seperate vfsmount for each subvolume? Thanks, >
What are the drawbacks of having a vfsmount for each subvolume? Why (besides having to code it up) are you trying to avoid doing it that way? -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
