From dadced3044015cbd80dc3b52d3f2f55039f2806f Mon Sep 17 00:00:00 2001 From: Olaf Kirch Date: Wed, 9 May 2012 14:12:16 -0400 Subject: [PATCH] Manpage: 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 upstream): https://build.opensuse.org/package/view_file?file=warn-nfs-udp.patch&package=nfs-utils&project=openSUSE%3AFactory&rev=8e3e60c70e8270cd4afa036e13f6b2bb Signed-off-by: Harshula Jayasuriya Acked-by: Chuck Lever Signed-off-by: Olaf Kirch Signed-off-by: Steve Dickson --- utils/mount/nfs.man | 81 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 81 insertions(+) diff --git a/utils/mount/nfs.man b/utils/mount/nfs.man index 0d20cf0..87e27e1 100644 --- a/utils/mount/nfs.man +++ b/utils/mount/nfs.man @@ -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. .IP +.B Before using NFS over UDP, refer to the TRANSPORT METHODS section. +.IP 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. +.IP +.B Before using NFS over UDP, refer to the TRANSPORT METHODS section. .TP 1.5i .B tcp The @@ -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" . +.P +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. +.P +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 is +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. +.P +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. +.P +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 packet. +.P +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. +.P +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. +.P +It is therefore strongly recommended to use +.BR "NFS over TCP where possible" , +since TCP does not perform fragmentation. +.P +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. +.IP +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 . +.IP +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. .SH "DATA AND METADATA COHERENCE" Some modern cluster file systems provide perfect cache coherence among their clients. -- 2.39.2