nfs: add FAQ section to Documentation/filesystems/nfs/localio.rst

Add a FAQ section to give answers to questions that have been raised
during review of the localio feature.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Co-developed-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Reviewed-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

Author: Trond Myklebust

Documentation/filesystems/nfs/localio.rst

@@ -64,6 +64,92 @@ fio for 20 secs with directio, qd of 8, 1 libaio thread:
   128K read:  IOPS=24.4k, BW=3050MiB/s (3198MB/s)(59.6GiB/20001msec)
   128K write: IOPS=11.4k, BW=1430MiB/s (1500MB/s)(27.9GiB/20001msec)
 
+FAQ
+===
+
+1. What are the use cases for LOCALIO?
+
+   a. Workloads where the NFS client and server are on the same host
+      realize improved IO performance. In particular, it is common when
+      running containerised workloads for jobs to find themselves
+      running on the same host as the knfsd server being used for
+      storage.
+
+2. What are the requirements for LOCALIO?
+
+   a. Bypass use of the network RPC protocol as much as possible. This
+      includes bypassing XDR and RPC for open, read, write and commit
+      operations.
+   b. Allow client and server to autonomously discover if they are
+      running local to each other without making any assumptions about
+      the local network topology.
+   c. Support the use of containers by being compatible with relevant
+      namespaces (e.g. network, user, mount).
+   d. Support all versions of NFS. NFSv3 is of particular importance
+      because it has wide enterprise usage and pNFS flexfiles makes use
+      of it for the data path.
+
+3. Why doesn’t LOCALIO just compare IP addresses or hostnames when
+   deciding if the NFS client and server are co-located on the same
+   host?
+
+   Since one of the main use cases is containerised workloads, we cannot
+   assume that IP addresses will be shared between the client and
+   server. This sets up a requirement for a handshake protocol that
+   needs to go over the same connection as the NFS traffic in order to
+   identify that the client and the server really are running on the
+   same host. The handshake uses a secret that is sent over the wire,
+   and can be verified by both parties by comparing with a value stored
+   in shared kernel memory if they are truly co-located.
+
+4. Does LOCALIO improve pNFS flexfiles?
+
+   Yes, LOCALIO complements pNFS flexfiles by allowing it to take
+   advantage of NFS client and server locality.  Policy that initiates
+   client IO as closely to the server where the data is stored naturally
+   benefits from the data path optimization LOCALIO provides.
+
+5. Why not develop a new pNFS layout to enable LOCALIO?
+
+   A new pNFS layout could be developed, but doing so would put the
+   onus on the server to somehow discover that the client is co-located
+   when deciding to hand out the layout.
+   There is value in a simpler approach (as provided by LOCALIO) that
+   allows the NFS client to negotiate and leverage locality without
+   requiring more elaborate modeling and discovery of such locality in a
+   more centralized manner.
+
+6. Why is having the client perform a server-side file OPEN, without
+   using RPC, beneficial?  Is the benefit pNFS specific?
+
+   Avoiding the use of XDR and RPC for file opens is beneficial to
+   performance regardless of whether pNFS is used. Especially when
+   dealing with small files its best to avoid going over the wire
+   whenever possible, otherwise it could reduce or even negate the
+   benefits of avoiding the wire for doing the small file I/O itself.
+   Given LOCALIO's requirements the current approach of having the
+   client perform a server-side file open, without using RPC, is ideal.
+   If in the future requirements change then we can adapt accordingly.
+
+7. Why is LOCALIO only supported with UNIX Authentication (AUTH_UNIX)?
+
+   Strong authentication is usually tied to the connection itself. It
+   works by establishing a context that is cached by the server, and
+   that acts as the key for discovering the authorisation token, which
+   can then be passed to rpc.mountd to complete the authentication
+   process. On the other hand, in the case of AUTH_UNIX, the credential
+   that was passed over the wire is used directly as the key in the
+   upcall to rpc.mountd. This simplifies the authentication process, and
+   so makes AUTH_UNIX easier to support.
+
+8. How do export options that translate RPC user IDs behave for LOCALIO
+   operations (eg. root_squash, all_squash)?
+
+   Export options that translate user IDs are managed by nfsd_setuser()
+   which is called by nfsd_setuser_and_check_port() which is called by
+   __fh_verify().  So they get handled exactly the same way for LOCALIO
+   as they do for non-LOCALIO.
+
 RPC
 ===