On Wed, 2009-06-03 at 13:22 -0300, Carlos Carvalho wrote: > Trond Myklebust (trond.myklebust@xxxxxxxxxx) wrote on 2 June 2009 13:27: > >Write gathering relies on waiting an arbitrary length of time in order > >to see if someone is going to send another write. The protocol offers no > >guidance as to how long that wait should be, and so (at least on the > >Linux server) we've coded in a hard wait of 10ms if and only if we see > >that something else has the file open for writing. > >One problem with the Linux implementation is that the "something else" > >could be another nfs server thread that happens to be in nfsd_write(), > >however it could also be another open NFSv4 stateid, or a NLM lock, or a > >local process that has the file open for writing. > >Another problem is that the nfs server keeps a record of the last file > >that was accessed, and also waits if it sees you are writing again to > >that same file. Of course it has no idea if this is truly a parallel > >write, or if it just happens that you are writing again to the same file > >using O_SYNC... > > I think the decision to write or wait doesn't belong to the nfs > server; it should just send the writes immediately. It's up to the > fs/block/device layers to do the gathering. I understand that the > client should try to do the gathering before sending the request to > the wire This isn't something that we've just pulled out of a hat. It dates back to pre-NFSv3 times, when every write had to be synchronously committed to disk before the RPC call could return. See, for instance, http://books.google.com/books?id=y9GgPhjyOUwC&pg=PA243&lpg=PA243&dq=What +is+nfs+write +gathering&source=bl&ots=M8s0XS2SLd&sig=ctmxQrpII2_Ti4czgpGZrF9mmds&hl=en&ei=Xa0mSrLMC8iptgfSsqHsBg&sa=X&oi=book_result&ct=result&resnum=3 The point is that while it is a good idea for NFSv2, we have much better methods of dealing with multiple writes in NFSv3 and v4... Trond -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
