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

  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

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