nfsiod, biod - The local NFS compatible asynchronous I/O
The nfsiod daemon runs on an NFS compatible client machine
and spawns several IO threads to service asynchronous I/O
requests to its server. The I/O threads improve performance
of both NFS reads and writes. Both try to enlist
the aid of an idle I/O thread. If none is available, the
process itself issues the request to the server and waits
for the reply.
The optimum number of I/O threads to run depends on many
variables, such as how quickly the client will be writing,
how many files will be accessed simultaneously, and the
behaviour of the NFS server. For use with a Tru64 UNIX
server, 7 is a good number of I/O threads for most systems.
When reading, if the client believes the process is reading
a file sequentially, it requests an I/O thread to read
a block ahead of what the process is currently requesting.
If the readahead completes before the process asks for
that block, then the subsequent read system call for that
data completes immediately and does not have to wait for
the NFS request to complete. Read ahead will be triggered
again so the read may find that next block available as
When writing a file, the client takes the process's data,
passes the request to an I/O thread and immediately
returns to the process. If the process is writing data
faster than the network or server can process, then eventually
all the I/O threads become busy and the process has
to handle a NFS write itself. This means the process has
to wait until the server finishes the write. For Tru64
UNIX servers, the NFS block size is 8Kb and UFS tries to
cluster I/O 64Kbs at a time. If the client is running
with 7 I/O threads, 8 write requests can be in progress at
once. This allows the client and server to write data
64Kbs at a time and is the reason for recommending 7 I/O
Unlike nfsd, each client thread can use either UDP or TCP.
However, if TCP mounts are active, the nfsiod process will
time out, close idle TCP connections, and acknowledge any
connections closed by the server.
The nfsiod process is also responsible for syncing the
access time and modify times for special files and named
pipes (fifos). Because I/O to these files does not go
through the NFS server, NFS clients have to directly
update the access time and modify time attributes.
The client threads are implemented as kernel threads; they
are part of Process ID 0, not the nfsiod process. The ps
axml command displays idle I/O threads under PID 0. Idle
threads will be waiting on nfsiod_wait. Therefore, if 7
I/O threads are configured, only 1 nfsiod process is displayed
in the output from the ps command, although 7
client threads are available to handle NFS requests.
Specifies the command path Specifies the file for logging
Commands: nfsd(8), nfsstat(8)
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