119 lines
4.8 KiB
Diff
119 lines
4.8 KiB
Diff
Index: util-linux-2.12/mount/nfs.5
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================================================================================
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--- util-linux-2.12q/mount/nfs.5
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+++ util-linux-2.12q/mount/nfs.5
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@@ -201,9 +201,13 @@
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Mount the NFS filesystem using the TCP protocol instead of the
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UDP protocol. This is the default, but in case it fails (many NFS servers only
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support UDP) it will fallback and try UDP.
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+.IP
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+.B Before using NFS over UDP, please refer to the section WARNINGS below.
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.TP 1.5i
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.I udp
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Mount the NFS filesystem using the UDP protocol.
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+.IP
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+.B Before using NFS over UDP, please refer to the section WARNINGS below.
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.TP 1.5i
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.I noacl
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Assume no extended access control mechanisms like POSIX ACLs are used
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@@ -217,6 +221,83 @@
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All of the non-value options have corresponding nooption forms.
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For example, nointr means don't allow file operations to be
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interrupted.
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+.SH WARNINGS
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+Using NFS over UDP on high-speed links such as Gigabit
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+.BR "can cause silent data corruption" .
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+.P
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+The problem can be triggered at high loads, and is caused by problems in
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+IP fragment reassembly. NFS read and writes typically transmit UDP packets
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+of 4 Kilobytes or more, which have to be broken up into several fragments
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+in order to be sent over the Ethernet link, which limits packets to 1500
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+bytes by default. This process happens at the IP network layer and is
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+called fragmentation.
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+.P
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+In order to identify fragments that belong together, IP assigns a 16bit
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+.I IP ID
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+value to each packet; fragments generated from the same UDP packet
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+will have the same IP ID. The receiving system will collect these
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+fragments and combine them to form the original UDP packet. This process
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+is called reassembly. The default timeout for packet reassembly is
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+30 seconds; if the network stack does not receive all fragments of
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+a given packet within this interval, it assumes the missing fragment(s)
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+got lost and discards those it already received.
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+.P
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+The problem this creates over high-speed links is that it is possible
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+to send more than 65536 packets within 30 seconds. In fact, with
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+heavy NFS traffic one can observe that the IP IDs repeat after about
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+5 seconds.
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+.P
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+This has serious effects on reassembly: if one fragment gets lost,
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+another fragment
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+.I from a different packet
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+but with the
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+.I same IP ID
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+will arrive within the 30 second timeout, and the network stack will
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+combine these fragments to form a new packet. Most of the time, network
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+layers above IP will detect this mismatched reassembly - in the case
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+of UDP, the UDP checksum, which is a 16 bit checksum over the entire
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+packet payload, will usually not match, and UDP will discard the
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+bad packet.
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+.P
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+However, the UDP checksum is 16 bit only, so there is a chance of 1 in
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+65536 that it will match even if the packet payload is completely
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+random (which very often isn't the case). If that is the case,
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+silent data corruption will occur.
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+.P
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+This potential should be taken seriously, at least on Gigabit
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+Ethernet.
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+Network speeds of 100Mbit/s should be considered less
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+problematic, because with most traffic patterns IP ID wrap around
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+will take much longer than 30 seconds.
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+.P
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+It is therefore strongly recommended to use
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+.BR "NFS over TCP where possible" ,
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+since TCP does not perform fragmentation.
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+.P
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+If you absolutely have to use NFS over UDP over Gigabit Ethernet,
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+some steps can be taken to mitigate the problem and reduce the
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+probability of corruption:
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+.TP +1.5i
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+.I Jumbo frames:
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+Many Gigabit network cards are capable of transmitting
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+frames bigger than the 1500 byte limit of traditional Ethernet, typically
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+9000 bytes. Using jumbo frames of 9000 bytes will allow you to run NFS over
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+UDP at a page size of 8K without fragmentation. Of course, this is
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+only feasible if all involved stations support jumbo frames.
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+.IP
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+To enable a machine to send jumbo frames on cards that support it,
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+it is sufficient to configure the interface for a MTU value of 9000.
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+.TP +1.5i
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+.I Lower reassembly timeout:
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+By lowering this timeout below the time it takes the IP ID counter
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+to wrap around, incorrect reassembly of fragments can be prevented
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+as well. To do so, simply write the new timeout value (in seconds)
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+to the file
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+.BR /proc/sys/net/ipv4/ipfrag_time .
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+.IP
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+A value of 2 seconds will greatly reduce the probability of IPID clashes on
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+a single Gigabit link, while still allowing for a reasonable timeout
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+when receiving fragmented traffic from distant peers.
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.SH FILES
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.I /etc/fstab
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.SH "SEE ALSO"
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--- util-linux-2.12q/mount/nfsmount.c
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+++ util-linux-2.12q/mount/nfsmount.c
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@@ -737,8 +737,12 @@
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goto fail;
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}
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fsock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
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- } else
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+ } else {
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fsock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
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+ fprintf(stderr,
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+ "Using NFS over UDP can cause data corruption.\n"
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+ "Please refer to the WARNINGS section of the nfs(5) manual page.\n");
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+ }
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if (fsock < 0) {
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perror(_("nfs socket"));
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goto fail;
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