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this read-ahead.) The client kernel is also running multiple NFS block I/O daemons
(biod processes) that try to generate multiple RPC requests on behalf of clients. One
daemon is reading 8192 bytes starting at 65536 (in 1024-byte chunks) and the other is
performing the read-ahead of 8192 bytes starting at 73728.
Client retransmissions occur in lines 130-168. In line 169 we see the server has rebooted,
and it sends an ARP request before it can reply to the client's NFS request in line 168. The
response to line 168 is sent in line 171. The client READ requests continue.
The client application never knows that the server crashes and reboots, and except for the
5-minute pause between lines 129 and 171, this server crash is transparent to the client.
To examine the timeout and retransmission interval in this example, realize that there are
two client daemons with their own timeouts. The intervals for the first daemon (reading at
offset 65536), rounded to two decimal points, are: 0.68, 0.87, 1.74, 3.48, 6.96, 13.92, 20.0,
20.0, 20.0, and so on. The intervals for the second daemon (reading at offset 73728) are
the same (to two decimal points). It appears that these NFS clients are using a timeout that
is a multiple of 0.875 seconds with an upper bound of 20 seconds. After each timeout the
retransmission interval is doubled: 0.875, 1.75,3.5, 7.0, and 14.0.
How long does the client retransmit? The client has two options that affect this. First, if
the server filesystem is mounted hard, the client retransmits forever, but if the server
filesystem is mounted soft, the client gives up after a fixed number of retransmissions.
Also, with a hard mount the client has an option of whether to let the user interrupt the
infinite retransmissions or not. If the client host specifies interruptibility when it mounts
the server's filesystem, if we don't want to wait 5 minutes for the server to reboot after it
crashes, we can type our interrupt key to abort the client application. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/nfs_netw.htm (20 of 23) [12/09/2001 14.47.56] C...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12