On 05/08/2012 08:59 PM, Harshula Jayasuriya wrote: > * 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 <harshula@xxxxxxxxxx> > Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx> > Signed-off-by: Olaf Kirch <okir@xxxxxxxx> Committed... steved. > --- > utils/mount/nfs.man | 81 +++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 files changed, 81 insertions(+), 0 deletions(-) > > 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. > -- 1.7.7.6 > -- 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
