Filesystem hard links

[penberg]

OSv does not support hard links that are needed by Apache Cassandra for snapshotting, for example.

Christoph Hellwig already did vnode and dentry separation that's in master and the remaining TODO is:

• wire up the vnode hash again, this time with a uin64_t as index
• use the znode id as index for zfs, a counter for ramfs/devfs
• write the actual link syscall and vnop (should be trivial)
• lots of testing

[penberg]

As pointed out by @raphaelsc, at least fs/vfs/vfs_vnode.c vn_lookup() still has the 1:1 mapping for mount/path and vnode. The function is not used and looks like left-over from hch's dentry conversion.

[penberg]

ZFS supports hard links now.

[penberg]

AFAICT, the VFS namei() function was broken by the hard link patches. It now does a vget() if dentry is not found which allocates a new vnode. What we should do instead is wire up a vnode cache and make a lookup based on inode number; otherwise we'll have more than one instances of a vnode in memory which is wrong.

[raphaelsc]

Please, close this issue. vnode duplication issue was already fixed by the commit e4aad1b