[Gluster-devel] Minimal Design Doc for Name Space Cache

[Krishna Srinivas]

Minimal Design Doc for Name Space Cache:
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Name Space Cache is a directory tree structure of the GlusterFS but with
empty files (not empty, file will contain the IP address of the server
where the file exists)
This helps us in two issues:
1) In case a server goes down, there are chances that a duplicate file name
    is created (creation will be allowed as the original file is not seen as the
    server has gone down) But if we have NSC info, we can make sure that
    duplicate file is not created.
2) open() will be faster as the NSC will have the info of the server where the
    file exists.

NSC has two components, server and client. NSC Server can be run on one
of the server nodes (nscd --root /home/nsc --port 7000) (or should it be a part
of glusterfsd?). NSC client module can be a part of glusterfs. client vol spec
can contain the line "name-space-cache-server <IP> <port>" in unify volume.

NSC client module will give the following functions to glusterfs
(these functions
will mostly be used by unify xlator)
nsc_init(IP, port)
nsc_fini()
nsc_query(path) - returns the IP addr of the node where file exists.
nsc_create(path, IP) called during creation of file
nsc_unlink(path)
nsc_rename(oldpath, newpath, newIP)

Unify create() calls nsc_create()
Unify unlink() will call nsc_unlink()
Unify rename() will call nsc_rename()

Unify init() or glusterfs init() will call nsc_init()
Unify fini() or glusterfs fini() will call nsc_fini()

Unify open() will call nsc_query() to get the IP address of the node where
the file exits. Then it will query all its child xlator to see which of them is
associated with that IP address and call open on that xlator. (This can be
implemented by introducing an mop function?)

Comments and suggestions please.