[Lustre-discuss] write RPC & congestion

[burlen]

Hi, thanks for previous help.

I have some question about Lustre RPC and the sequence of events that 
occur during large concurrent write() involving many processes and large 
data size per process.  I understand there is a mechanism of flow 
control by credits, but I'm a little unclear on how it works in general 
after reading the "networking & io protocol" white paper.

Is it true that a write() RPC transfer's data in chunks of at least 1MB 
and at most (max_pages_per_rpc*page_size) Bytes, where page_size=2^16 ? 
I can use the bounds to estimate the number of RPCs issued per MB of 
data to write?

About how many concurrent incoming write() RPC per OSS service thread 
can a single server handle before it stops responding to incoming RPCs ?

What happens to an RPC when the server is too busy to handle it, is it 
even issued by the client ? Does the client have to poll and/or resend 
the RPC ? Does the process of polling for flow control credits add 
significant network/server congestion ?

Is it likely that a large number of RPC's/flow control credit requests 
will induce enough network congestion so that client's RPC's timeout ? 
How does the client handle such a timeout ?

Burlen