RE: [PATCH] nfs-utils: Add a warning to the nfs manpage regarding using NFS over UDP on high-speed links

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I thought that we had previously discussed whether or not to include this sort of text and had come to the conclusion to not include it because the problem is not new or unique to NFS.  It is a general networking issue.  Am I remembering incorrectly?



-----Original Message-----
From: linux-nfs-owner@xxxxxxxxxxxxxxx [mailto:linux-nfs-owner@xxxxxxxxxxxxxxx] On Behalf Of Harshula Jayasuriya
Sent: Tuesday, May 08, 2012 8:59 PM
To: Steve Dickson
Cc: Jeff Layton; Linux NFS Mailing List; Chuck Lever; Olaf Kirch
Subject: [PATCH] nfs-utils: Add a warning to the nfs manpage regarding using NFS over UDP on high-speed links

* Using NFS over UDP on high-speed links such as Gigabit can cause
  silent data corruption.
* The man page text was written by Olaf Kirch and committed to (but not

Signed-off-by: Harshula Jayasuriya <harshula@xxxxxxxxxx>
Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
Signed-off-by: Olaf Kirch <okir@xxxxxxxx>
 utils/mount/ |   81 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 81 insertions(+), 0 deletions(-)

diff --git a/utils/mount/ b/utils/mount/ index 0d20cf0..87e27e1 100644
--- a/utils/mount/
+++ b/utils/mount/
@@ -500,6 +500,8 @@ Specifying a netid that uses TCP forces all traffic from the  command and the NFS client to use TCP.
 Specifying a netid that uses UDP forces all traffic types to use UDP.
+.B Before using NFS over UDP, refer to the TRANSPORT METHODS section.
 If the
 .B proto
 mount option is not specified, the
@@ -514,6 +516,8 @@ The
 option is an alternative to specifying
 .BR proto=udp.
 It is included for compatibility with other operating systems.
+.B Before using NFS over UDP, refer to the TRANSPORT METHODS section.
 .TP 1.5i
 .B tcp
@@ -1070,6 +1074,83 @@ or
 options are specified more than once on the same mount command line,  then the value of the rightmost instance of each of these options  takes effect.
+.SS "Using NFS over UDP on high-speed links"
+Using NFS over UDP on high-speed links such as Gigabit .BR "can cause 
+silent data corruption" .
+The problem can be triggered at high loads, and is caused by problems 
+in IP fragment reassembly. NFS read and writes typically transmit UDP 
+packets of 4 Kilobytes or more, which have to be broken up into several 
+fragments in order to be sent over the Ethernet link, which limits 
+packets to 1500 bytes by default. This process happens at the IP 
+network layer and is called fragmentation.
+In order to identify fragments that belong together, IP assigns a 16bit 
+.I IP ID value to each packet; fragments generated from the same UDP 
+packet will have the same IP ID. The receiving system will collect 
+these fragments and combine them to form the original UDP packet. This 
+process is called reassembly. The default timeout for packet reassembly 
+30 seconds; if the network stack does not receive all fragments of a 
+given packet within this interval, it assumes the missing fragment(s) 
+got lost and discards those it already received.
+The problem this creates over high-speed links is that it is possible 
+to send more than 65536 packets within 30 seconds. In fact, with heavy 
+NFS traffic one can observe that the IP IDs repeat after about
+5 seconds.
+This has serious effects on reassembly: if one fragment gets lost, 
+another fragment .I from a different packet but with the .I same IP ID 
+will arrive within the 30 second timeout, and the network stack will 
+combine these fragments to form a new packet. Most of the time, network 
+layers above IP will detect this mismatched reassembly - in the case of 
+UDP, the UDP checksum, which is a 16 bit checksum over the entire 
+packet payload, will usually not match, and UDP will discard the bad 
+However, the UDP checksum is 16 bit only, so there is a chance of 1 in
+65536 that it will match even if the packet payload is completely 
+random (which very often isn't the case). If that is the case, silent 
+data corruption will occur.
+This potential should be taken seriously, at least on Gigabit Ethernet.
+Network speeds of 100Mbit/s should be considered less problematic, 
+because with most traffic patterns IP ID wrap around will take much 
+longer than 30 seconds.
+It is therefore strongly recommended to use .BR "NFS over TCP where 
+possible" , since TCP does not perform fragmentation.
+If you absolutely have to use NFS over UDP over Gigabit Ethernet, some 
+steps can be taken to mitigate the problem and reduce the probability 
+of corruption:
+.TP +1.5i
+.I Jumbo frames:
+Many Gigabit network cards are capable of transmitting frames bigger 
+than the 1500 byte limit of traditional Ethernet, typically
+9000 bytes. Using jumbo frames of 9000 bytes will allow you to run NFS 
+over UDP at a page size of 8K without fragmentation. Of course, this is 
+only feasible if all involved stations support jumbo frames.
+To enable a machine to send jumbo frames on cards that support it, it 
+is sufficient to configure the interface for a MTU value of 9000.
+.TP +1.5i
+.I Lower reassembly timeout:
+By lowering this timeout below the time it takes the IP ID counter to 
+wrap around, incorrect reassembly of fragments can be prevented as 
+well. To do so, simply write the new timeout value (in seconds) to the 
+file .BR /proc/sys/net/ipv4/ipfrag_time .
+A value of 2 seconds will greatly reduce the probability of IPID 
+clashes on a single Gigabit link, while still allowing for a reasonable 
+timeout when receiving fragmented traffic from distant peers.
 Some modern cluster file systems provide  perfect cache coherence among their clients.

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