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diff --git a/share/doc/smm/06.nfs/1.t b/share/doc/smm/06.nfs/1.t new file mode 100644 index 0000000..6804ed1 --- /dev/null +++ b/share/doc/smm/06.nfs/1.t @@ -0,0 +1,588 @@ +.\" Copyright (c) 1993 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" This document is derived from software contributed to Berkeley by +.\" Rick Macklem at The University of Guelph. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 3. All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" This product includes software developed by the University of +.\" California, Berkeley and its contributors. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)1.t 8.1 (Berkeley) 6/8/93 +.\" +.sh 1 "NFS Implementation" +.pp +The 4.4BSD implementation of NFS and the alternate protocol nicknamed +Not Quite NFS (NQNFS) are kernel resident, but make use of a few system +daemons. +The kernel implementation does not use an RPC library, handling the RPC +request and reply messages directly in \fImbuf\fR data areas. NFS +interfaces to the network using +sockets via. the kernel interface available in +\fIsys/kern/uipc_syscalls.c\fR as \fIsosend(), soreceive(),\fR... +There are connection management routines for support of sockets for connection +oriented protocols and timeout/retransmit support for datagram sockets on +the client side. +For connection oriented transport protocols, +such as TCP/IP, there is one connection +for each client to server mount point that is maintained until an umount. +If the connection breaks, the client will attempt a reconnect with a new +socket. +The client side can operate without any daemons running, but performance +will be improved by running nfsiod daemons that perform read-aheads +and write-behinds. +For the server side to function, the daemons portmap, mountd and +nfsd must be running. +The mountd daemon performs two important functions. +.ip 1) +Upon startup and after a hangup signal, mountd reads the exports +file and pushes the export information for each local file system down +into the kernel via. the mount system call. +.ip 2) +Mountd handles remote mount protocol (RFC1094, Appendix A) requests. +.lp +The nfsd master daemon forks off children that enter the kernel +via. the nfssvc system call. The children normally remain kernel +resident, providing a process context for the NFS RPC servers. The only +exception to this is when a Kerberos [Steiner88] +ticket is received and at that time +the nfsd exits the kernel temporarily to verify the ticket via. the +Kerberos libraries and then returns to the kernel with the results. +(This only happens for Kerberos mount points as described further under +Security.) +Meanwhile, the master nfsd waits to accept new connections from clients +using connection oriented transport protocols and passes the new sockets down +into the kernel. +The client side mount_nfs along with portmap and +mountd are the only parts of the NFS subsystem that make any +use of the Sun RPC library. +.sh 1 "Mount Problems" +.pp +There are several problems that can be encountered at the time of an NFS +mount, ranging from a unresponsive NFS server (crashed, network partitioned +from client, etc.) to various interoperability problems between different +NFS implementations. +.pp +On the server side, +if the 4.4BSD NFS server will be handling any PC clients, mountd will +require the \fB-n\fR option to enable non-root mount request servicing. +Running of a pcnfsd\** daemon will also be necessary. +.(f +\** Pcnfsd is available in source form from Sun Microsystems and many +anonymous ftp sites. +.)f +The server side requires that the daemons +mountd and nfsd be running and that +they be registered with portmap properly. +If problems are encountered, +the safest fix is to kill all the daemons and then restart them in +the order portmap, mountd and nfsd. +Other server side problems are normally caused by problems with the format +of the exports file, which is covered under +Security and in the exports man page. +.pp +On the client side, there are several mount options useful for dealing +with server problems. +In cases where a file system is not critical for system operation, the +\fB-b\fR +mount option may be specified so that mount_nfs will go into the +background for a mount attempt on an unresponsive server. +This is useful for mounts specified in +\fIfstab(5)\fR, +so that the system will not get hung while booting doing +\fBmount -a\fR +because a file server is not responsive. +On the other hand, if the file system is critical to system operation, this +option should not be used so that the client will wait for the server to +come up before completing bootstrapping. +There are also three mount options to help deal with interoperability issues +with various non-BSD NFS servers. The +\fB-P\fR +option specifies that the NFS +client use a reserved IP port number to satisfy some servers' security +requirements.\** +.(f +\**Any security benefit of this is highly questionable and as +such the BSD server does not require a client to use a reserved port number. +.)f +The +\fB-c\fR +option stops the NFS client from doing a \fIconnect\fR on the UDP +socket, so that the mount works with servers that send NFS replies from +port numbers other than the standard 2049.\** +.(f +\**The Encore Multimax is known +to require this. +.)f +Finally, the +\fB-g=\fInum\fR +option sets the maximum size of the group list in the credentials passed +to an NFS server in every RPC request. Although RFC1057 specifies a maximum +size of 16 for the group list, some servers can't handle that many. +If a user, particularly root doing a mount, +keeps getting access denied from a file server, try temporarily +reducing the number of groups that user is in to less than 5 +by editing /etc/group. If the user can then access the file system, slowly +increase the number of groups for that user until the limit is found and +then peg the limit there with the +\fB-g=\fInum\fR +option. +This implies that the server will only see the first \fInum\fR +groups that the user is in, which can cause some accessibility problems. +.pp +For sites that have many NFS servers, amd [Pendry93] +is a useful administration tool. +It also reduces the number of actual NFS mount points, alleviating problems +with commands such as df(1) that hang when any of the NFS servers is +unreachable. +.sh 1 "Dealing with Hung Servers" +.pp +There are several mount options available to help a client deal with +being hung waiting for response from a crashed or unreachable\** server. +.(f +\**Due to a network partitioning or similar. +.)f +By default, a hard mount will continue to try to contact the server +``forever'' to complete the system call. This type of mount is appropriate +when processes on the client that access files in the file system do not +tolerate file I/O systems calls that return -1 with \fIerrno == EINTR\fR +and/or access to the file system is critical for normal system operation. +.lp +There are two other alternatives: +.ip 1) +A soft mount (\fB-s\fR option) retries an RPC \fIn\fR +times and then the corresponding +system call returns -1 with errno set to EINTR. +For TCP transport, the actual RPC request is not retransmitted, but the +timeout intervals waiting for a reply from the server are done +in the same manner as UDP for this purpose. +The problem with this type of mount is that most applications do not +expect an EINTR error return from file I/O system calls (since it never +occurs for a local file system) and get confused by the error return +from the I/O system call. +The option +\fB-x=\fInum\fR +is used to set the RPC retry limit and if set too low, the error returns +will start occurring whenever the NFS server is slow due to heavy load. +Alternately, a large retry limit can result in a process hung for a long +time, due to a crashed server or network partitioning. +.ip 2) +An interruptible mount (\fB-i\fR option) checks to see if a termination signal +is pending for the process when waiting for server response and if it is, +the I/O system call posts an EINTR. Normally this results in the process +being terminated by the signal when returning from the system call. +This feature allows you to ``^C'' out of processes that are hung +due to unresponsive servers. +The problem with this approach is that signals that are caught by +a process are not recognized as termination signals +and the process will remain hung.\** +.(f +\**Unfortunately, there are also some resource allocation situations in the +BSD kernel where the termination signal will be ignored and the process +will not terminate. +.)f +.sh 1 "RPC Transport Issues" +.pp +The NFS Version 2 protocol runs over UDP/IP transport by +sending each Sun Remote Procedure Call (RFC1057) +request/reply message in a single UDP +datagram. Since UDP does not guarantee datagram delivery, the +Remote Procedure Call (RPC) layer +times out and retransmits an RPC request if +no RPC reply has been received. Since this round trip timeout (RTO) value +is for the entire RPC operation, including RPC message transmission to the +server, queuing at the server for an nfsd, performing the RPC and +sending the RPC reply message back to the client, it can be highly variable +for even a moderately loaded NFS server. +As a result, the RTO interval must be a conservation (large) estimate, in +order to avoid extraneous RPC request retransmits.\** +.(f +\**At best, an extraneous RPC request retransmit increases +the load on the server and at worst can result in damaged files +on the server when non-idempotent RPCs are redone [Juszczak89]. +.)f +Also, with an 8Kbyte read/write data size +(the default), the read/write reply/request will be an 8+Kbyte UDP datagram +that must normally be fragmented at the IP layer for transmission.\** +.(f +\**6 IP fragments for an Ethernet, +which has an maximum transmission unit of 1500bytes. +.)f +For IP fragments to be successfully reassembled into +the IP datagram at the receive end, all +fragments must be received within a fairly short ``time to live''. +If one fragment is lost/damaged in transit, +the entire RPC must be retransmitted and redone. +This problem can be exaggerated by a network interface on the receiver that +cannot handle the reception of back to back network packets. [Kent87a] +.pp +There are several tuning mount +options on the client side that can prove useful when trying to +alleviate performance problems related to UDP RPC transport. +The options +\fB-r=\fInum\fR +and +\fB-w=\fInum\fR +specify the maximum read or write data size respectively. +The size \fInum\fR +should be a power of 2 (4K, 2K, 1K) and adjusted downward from the +maximum of 8Kbytes +whenever IP fragmentation is causing problems. The best indicator of +IP fragmentation problems is a significant number of +\fIfragments dropped after timeout\fR +reported by the \fIip:\fR section of a \fBnetstat -s\fR +command on either the client or server. +Of course, if the fragments are being dropped at the server, it can be +fun figuring out which client(s) are involved. +The most likely candidates are clients that are not +on the same local area network as the +server or have network interfaces that do not receive several +back to back network packets properly. +.pp +By default, the 4.4BSD NFS client dynamically estimates the retransmit +timeout interval for the RPC and this appears to work reasonably well for +many environments. However, the +\fB-d\fR +flag can be specified to turn off +the dynamic estimation of retransmit timeout, so that the client will +use a static initial timeout interval.\** +.(f +\**After the first retransmit timeout, the initial interval is backed off +exponentially. +.)f +The +\fB-t=\fInum\fR +option can be used with +\fB-d\fR +to set the initial timeout interval to other than the default of 2 seconds. +The best indicator that dynamic estimation should be turned off would +be a significant number\** in the \fIX Replies\fR field and a +.(f +\**Even 0.1% of the total RPCs is probably significant. +.)f +large number in the \fIRetries\fR field +in the \fIRpc Info:\fR section as reported +by the \fBnfsstat\fR command. +On the server, there would be significant numbers of \fIInprog\fR recent +request cache hits in the \fIServer Cache Stats:\fR section as reported +by the \fBnfsstat\fR command, when run on the server. +.pp +The tradeoff is that a smaller timeout interval results in a better +average RPC response time, but increases the risk of extraneous retries +that in turn increase server load and the possibility of damaged files +on the server. It is probably best to err on the safe side and use a large +(>= 2sec) fixed timeout if the dynamic retransmit timeout estimation +seems to be causing problems. +.pp +An alternative to all this fiddling is to run NFS over TCP transport instead +of UDP. +Since the 4.4BSD TCP implementation provides reliable +delivery with congestion control, it avoids all of the above problems. +It also permits the use of read and write data sizes greater than the 8Kbyte +limit for UDP transport.\** +.(f +\**Read/write data sizes greater than 8Kbytes will not normally improve +performance unless the kernel constant MAXBSIZE is increased and the +file system on the server has a block size greater than 8Kbytes. +.)f +NFS over TCP usually delivers comparable to significantly better performance +than NFS over UDP +unless the client or server processor runs at less than 5-10MIPS. For a +slow processor, the extra CPU overhead of using TCP transport will become +significant and TCP transport may only be useful when the client +to server interconnect traverses congested gateways. +The main problem with using TCP transport is that it is only supported +between BSD clients and servers.\** +.(f +\**There are rumors of commercial NFS over TCP implementations on the horizon +and these may well be worth exploring. +.)f +.sh 1 "Other Tuning Tricks" +.pp +Another mount option that may improve performance over +certain network interconnects is \fB-a=\fInum\fR +which sets the number of blocks that the system will +attempt to read-ahead during sequential reading of a file. The default value +of 1 seems to be appropriate for most situations, but a larger value might +achieve better performance for some environments, such as a mount to a server +across a ``high bandwidth * round trip delay'' interconnect. +.pp +For the adventurous, playing with the size of the buffer cache +can also improve performance for some environments that use NFS heavily. +Under some workloads, a buffer cache of 4-6Mbytes can result in significant +performance improvements over 1-2Mbytes, both in client side system call +response time and reduced server RPC load. +The buffer cache size defaults to 10% of physical memory, +but this can be overridden by specifying the BUFPAGES option +in the machine's config file.\** +.(f +BUFPAGES is the number of physical machine pages allocated to the buffer cache. +ie. BUFPAGES * NBPG = buffer cache size in bytes +.)f +When increasing the size of BUFPAGES, it is also advisable to increase the +number of buffers NBUF by a corresponding amount. +Note that there is a tradeoff of memory allocated to the buffer cache versus +available for paging, which implies that making the buffer cache larger +will increase paging rate, with possibly disastrous results. +.sh 1 "Security Issues" +.pp +When a machine is running an NFS server it opens up a great big security hole. +For ordinary NFS, the server receives client credentials +in the RPC request as a user id +and a list of group ids and trusts them to be authentic! +The only tool available to restrict remote access to +file systems with is the exports(5) file, +so file systems should be exported with great care. +The exports file is read by mountd upon startup and after a hangup signal +is posted for it and then as much of the access specifications as possible are +pushed down into the kernel for use by the nfsd(s). +The trick here is that the kernel information is stored on a per +local file system mount point and client host address basis and cannot refer to +individual directories within the local server file system. +It is best to think of the exports file as referring to the various local +file systems and not just directory paths as mount points. +A local file system may be exported to a specific host, all hosts that +match a subnet mask or all other hosts (the world). The latter is very +dangerous and should only be used for public information. It is also +strongly recommended that file systems exported to ``the world'' be exported +read-only. +For each host or group of hosts, the file system can be exported read-only or +read/write. +You can also define one of three client user id to server credential +mappings to help control access. +Root (user id == 0) can be mapped to some default credentials while all other +user ids are accepted as given. +If the default credentials for user id equal zero +are root, then there is essentially no remapping. +Most NFS file systems are exported this way, most commonly mapping +user id == 0 to the credentials for the user nobody. +Since the client user id and group id list is used unchanged on the server +(except for root), this also implies that +the user id and group id space must be common between the client and server. +(ie. user id N on the client must refer to the same user on the server) +All user ids can be mapped to a default set of credentials, typically that of +the user nobody. This essentially gives world access to all +users on the corresponding hosts. +.pp +There is also a non-standard BSD +\fB-kerb\fR export option that requires the client provide +a KerberosIV rcmd service ticket to authenticate the user on the server. +If successful, the Kerberos principal is looked up in the server's password +and group databases to get a set of credentials and a map of client userid to +these credentials is then cached. +The use of TCP transport is strongly recommended, +since the scheme depends on the TCP connection to avert replay attempts. +Unfortunately, this option is only usable +between BSD clients and servers since it is +not compatible with other known ``kerberized'' NFS systems. +To enable use of this Kerberos option, both mount_nfs on the client and +nfsd on the server must be rebuilt with the -DKERBEROS option and +linked to KerberosIV libraries. +The file system is then exported to the client(s) with the \fB-kerb\fR option +in the exports file on the server +and the client mount specifies the +\fB-K\fR +and +\fB-T\fR +options. +The +\fB-m=\fIrealm\fR +mount option may be used to specify a Kerberos Realm for the ticket +(it must be the Kerberos Realm of the server) that is other than +the client's local Realm. +To access files in a \fB-kerb\fR mount point, the user must have a valid +TGT for the server's Realm, as provided by kinit or similar. +.pp +As well as the standard NFS Version 2 protocol (RFC1094) implementation, BSD +systems can use a variant of the protocol called Not Quite NFS (NQNFS) that +supports a variety of protocol extensions. +This protocol uses 64bit file offsets +and sizes, an \fIaccess rpc\fR, an \fIappend\fR option on the write rpc +and extended file attributes to support 4.4BSD file system functionality +more fully. +It also makes use of a variant of short term +\fIleases\fR [Gray89] with delayed write client caching, +in an effort to provide full cache consistency and better performance. +This protocol is available between 4.4BSD systems only and is used when +the \fB-q\fR mount option is specified. +It can be used with any of the aforementioned options for NFS, such as TCP +transport (\fB-T\fR) and KerberosIV authentication (\fB-K\fR). +Although this protocol is experimental, it is recommended over NFS for +mounts between 4.4BSD systems.\** +.(f +\**I would appreciate email from anyone who can provide +NFS vs. NQNFS performance measurements, +particularly fast clients, many clients or over an internetwork +connection with a large ``bandwidth * RTT'' product. +.)f +.sh 1 "Monitoring NFS Activity" +.pp +The basic command for monitoring NFS activity on clients and servers is +nfsstat. It reports cumulative statistics of various NFS activities, +such as counts of the various different RPCs and cache hit rates on the client +and server. Of particular interest on the server are the fields in the +\fIServer Cache Stats:\fR section, which gives numbers for RPC retries received +in the first three fields and total RPCs in the fourth. The first three fields +should remain a very small percentage of the total. If not, it +would indicate one or more clients doing retries too aggressively and the fix +would be to isolate these clients, +disable the dynamic RTO estimation on them and +make their initial timeout interval a conservative (ie. large) value. +.pp +On the client side, the fields in the \fIRpc Info:\fR section are of particular +interest, as they give an overall picture of NFS activity. +The \fITimedOut\fR field is the number of I/O system calls that returned -1 +for ``soft'' mounts and can be reduced +by increasing the retry limit or changing +the mount type to ``intr'' or ``hard''. +The \fIInvalid\fR field is a count of trashed RPC replies that are received +and should remain zero.\** +.(f +\**Some NFS implementations run with UDP checksums disabled, so garbage RPC +messages can be received. +.)f +The \fIX Replies\fR field counts the number of repeated RPC replies received +from the server and is a clear indication of a too aggressive RTO estimate. +Unfortunately, a good NFS server implementation will use a ``recent request +cache'' [Juszczak89] that will suppress the extraneous replies. +A large value for \fIRetries\fR indicates a problem, but +it could be any of: +.ip \(bu +a too aggressive RTO estimate +.ip \(bu +an overloaded NFS server +.ip \(bu +IP fragments being dropped (gateway, client or server) +.lp +and requires further investigation. +The \fIRequests\fR field is the total count of RPCs done on all servers. +.pp +The \fBnetstat -s\fR comes in useful during investigation of RPC transport +problems. +The field \fIfragments dropped after timeout\fR in +the \fIip:\fR section indicates IP fragments are +being lost and a significant number of these occurring indicates that the +use of TCP transport or a smaller read/write data size is in order. +A significant number of \fIbad checksums\fR reported in the \fIudp:\fR +section would suggest network problems of a more generic sort. +(cabling, transceiver or network hardware interface problems or similar) +.pp +There is a RPC activity logging facility for both the client and +server side in the kernel. +When logging is enabled by setting the kernel variable nfsrtton to +one, the logs in the kernel structures nfsrtt (for the client side) +and nfsdrt (for the server side) are updated upon the completion +of each RPC in a circular manner. +The pos element of the structure is the index of the next element +of the log array to be updated. +In other words, elements of the log array from \fIlog\fR[pos] to +\fIlog\fR[pos - 1] are in chronological order. +The include file <sys/nfsrtt.h> should be consulted for details on the +fields in the two log structures.\** +.(f +\**Unfortunately, a monitoring tool that uses these logs is still in the +planning (dreaming) stage. +.)f +.sh 1 "Diskless Client Support" +.pp +The NFS client does include kernel support for diskless/dataless operation +where the root file system and optionally the swap area is remote NFS mounted. +A diskless/dataless client is configured using a version of the +``swapvmunix.c'' file as provided in the directory \fIcontrib/diskless.nfs\fR. +If the swap device == NODEV, it specifies an NFS mounted swap area and should +be configured the same size as set up by diskless_setup when run on the server. +This file must be put in the \fIsys/compile/<machine_name>\fR kernel build +directory after the config command has been run, since config does +not know about specifying NFS root and swap areas. +The kernel variable mountroot must be set to nfs_mountroot instead of +ffs_mountroot and the kernel structure nfs_diskless must be filled in +properly. +There are some primitive system administration tools in the \fIcontrib/diskless.nfs\fR directory to assist in filling in +the nfs_diskless structure and in setting up an NFS server for +diskless/dataless clients. +The tools were designed to provide a bare bones capability, to allow maximum +flexibility when setting up different servers. +.lp +The tools are as follows: +.ip \(bu +diskless_offset.c - This little program reads a ``vmunix'' object file and +writes the file byte offset of the nfs_diskless structure in it to +standard out. It was kept separate because it sometimes has to +be compiled/linked in funny ways depending on the client architecture. +(See the comment at the beginning of it.) +.ip \(bu +diskless_setup.c - This program is run on the server and sets up files for a +given client. It mostly just fills in an nfs_diskless structure and +writes it out to either the "vmunix" file or a separate file called +/var/diskless/setup.<official-hostname> +.ip \(bu +diskless_boot.c - There are two functions in here that may be used +by a bootstrap server such as tftpd to permit sharing of the ``vmunix'' +object file for similar clients. This saves disk space on the bootstrap +server and simplify organization, but are not critical for correct operation. +They read the ``vmunix'' +file, but optionally fill in the nfs_diskless structure from a +separate "setup.<official-hostname>" file so that there is only +one copy of "vmunix" for all similar (same arch etc.) clients. +These functions use a text file called +/var/diskless/boot.<official-hostname> to control the netboot. +.lp +The basic setup steps are: +.ip \(bu +make a "vmunix" for the client(s) with mountroot() == nfs_mountroot() +and swdevt[0].sw_dev == NODEV if it is to do nfs swapping as well +(See the same swapvmunix.c file) +.ip \(bu +run diskless_offset on the vmunix file to find out the byte offset +of the nfs_diskless structure +.ip \(bu +Run diskless_setup on the server to set up the server and fill in the +nfs_diskless structure for that client. +The nfs_diskless structure can either be written into the +vmunix file (the -x option) or +saved in /var/diskless/setup.<official-hostname>. +.ip \(bu +Set up the bootstrap server. If the nfs_diskless structure was written into +the ``vmunix'' file, any vanilla bootstrap protocol such as bootp/tftp can +be used. If the bootstrap server has been modified to use the functions in +diskless_boot.c, then a +file called /var/diskless/boot.<official-hostname> +must be created. +It is simply a two line text file, where the first line is the pathname +of the correct ``vmunix'' file and the second line has the pathname of +the nfs_diskless structure file and its byte offset in it. +For example: +.br + /var/diskless/vmunix.pmax +.br + /var/diskless/setup.rickers.cis.uoguelph.ca 642308 +.br +.ip \(bu +Create a /var subtree for each client in an appropriate place on the server, +such as /var/diskless/var/<client-hostname>/... +By using the <client-hostname> to differentiate /var for each host, +/etc/rc can be modified to mount the correct /var from the server. |
