NFS Performance Tuning for HP-UX 11.0 and 11i Systems

nfs performance tuning for hp-ux 11.0 and 11i systems page 44

Notes:

Page 44July 22, 2002

nfsd

How do they work in the WRITE case?

1. Determine if the client is writing in synchronous or asynchronous

mode

2. Synchronous – schedule the WRITE to the underlying filesystem, sleep

until the call completes, then wake up and send a reply to the client

3. Asynchronous – post the data to cache and respond immediately

PV2 – Get the data posted to physical disk before the server crashes

PV3 – Get the data posted to physical disk before the client sends a

COMMIT, requiring the data to be flushed to stable storage.

GOAL: Post the data to the server’s buffer cache as quickly as

possible and allow the client to continue. Flush the data to

physical disk in the background as quickly as possible.

The nfsds (or kernel threads) behave differently depending up on whether the NFS

client is sending its write requests asking for synchronous or asynchronous

semantics.

In synchronous mode (the default behavior for NFS PV2), the nfsds take the write

data from the request and schedule a disk I/O operation to get the data posted to

stable storage as quickly as possible. The nfsd blocks waiting for the I/O to

complete before responding to the client and allowing it to continue.

In asynchronous mode (the default behavior for PV3, and can be enabled on PV2

by exporting the filesystem with the “–async” option), the nfsd takes the data from

the request, posts it to the server’s buffer cache, and sends a reply immediately

allowing the client process to continue before the data is actually posted to stable

storage. In the PV2 case, the NFS server’s goal is to get the data posted to

physical disk before the server experiences a system crash. In the PV3 case, the

NFS server tries to flush the data to stable storage before the client sends a

COMMIT request, since nfsd is not allowed to reply to a COMMIT request until all

data specified in the request is posted to physical disk.