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. 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.
@(#)2.t 8.1 (Berkeley) 6/8/93
$FreeBSD$
.sh 1 "Not Quite NFS, Crash Tolerant Cache Consistency for NFS" .pp Not Quite NFS (NQNFS) is an NFS like protocol designed to maintain full cache consistency between clients in a crash tolerant manner. It is an adaptation of the NFS protocol such that the server supports both NFS and NQNFS clients while maintaining full consistency between the server and NQNFS clients. This section borrows heavily from work done on Spritely-NFS [Srinivasan89], but uses Leases [Gray89] to avoid the need to recover server state information after a crash. The reader is strongly encouraged to read these references before trying to grasp the material presented here. .sh 2 "Overview" .pp The protocol maintains cache consistency by using a somewhat Sprite [Nelson88] like protocol, but is based on short term leases\** instead of hard state information about open files. .(f \** A lease is a ticket permitting an activity that is valid until some expiry time. .)f The basic principal is that the protocol will disable client caching of a file whenever that file is write shared\**. .(f \** Write sharing occurs when at least one client is modifying a file while other client(s) are reading the file. .)f Whenever a client wishes to cache data for a file it must hold a valid lease. There are three types of leases: read caching, write caching and non-caching. The latter type requires that all file operations be done synchronously with the server via. RPCs. A read caching lease allows for client data caching, but no file modifications may be done. A write caching lease allows for client caching of writes, but requires that all writes be pushed to the server when the lease expires. If a client has dirty buffers\** .(f \** Cached write data is not yet pushed (written) to the server. .)f when a write cache lease has almost expired, it will attempt to extend the lease but is required to push the dirty buffers if extension fails. A client gets leases by either doing a GetLease RPC or by piggybacking a GetLease Request onto another RPC. Piggybacking is supported for the frequent RPCs Getattr, Setattr, Lookup, Readlink, Read, Write and Readdir in an effort to minimize the number of GetLease RPCs required. All leases are at the granularity of a file, since all NFS RPCs operate on individual files and NFS has no intrinsic notion of a file hierarchy. Directories, symbolic links and file attributes may be read cached but are not write cached. The exception here is the attribute file_size, which is updated during cached writing on the client to reflect a growing file. .pp It is the server's responsibility to ensure that consistency is maintained among the NQNFS clients by disabling client caching whenever a server file operation would cause inconsistencies. The possibility of inconsistencies occurs whenever a client has a write caching lease and any other client, or local operations on the server, tries to access the file or when a modify operation is attempted on a file being read cached by client(s). At this time, the server sends an eviction notice to all clients holding the lease and then waits for lease termination. Lease termination occurs when a vacated the premises message has been received from all the clients that have signed the lease or when the lease expires via. timeout. The message pair eviction notice and vacated the premises roughly correspond to a Sprite server\(->client callback, but are not implemented as an actual RPC, to avoid the server waiting indefinitely for a reply from a dead client. .pp Server consistency checking can be viewed as issuing intrinsic leases for a file operation for the duration of the operation only. For example, the Create RPC will get an intrinsic write lease on the directory in which the file is being created, disabling client read caches for that directory. .pp By relegating this responsibility to the server, consistency between the server and NQNFS clients is maintained when NFS clients are modifying the file system as well.\** .(f \** The NFS clients will continue to be approximately consistent with the server. .)f .pp The leases are issued as time intervals to avoid the requirement of time of day clock synchronization. There are three important time constants known to the server. The maximum_lease_term sets an upper bound on lease duration. The clock_skew is added to all lease terms on the server to correct for differing clock speeds between the client and server and write_slack is the number of seconds the server is willing to wait for a client with an expired write caching lease to push dirty writes. .pp The server maintains a modify_revision number for each file. It is defined as an unsigned quadword integer that is never zero and that must increase whenever the corresponding file is modified on the server. It is used by the client to determine whether or not cached data for the file is stale. Generating this value is easier said than done. The current implementation uses the following technique, which is believed to be adequate. The high order longword is stored in the ufs inode and is initialized to one when an inode is first allocated. The low order longword is stored in main memory only and is initialized to zero when an inode is read in from disk. When the file is modified for the first time within a given second of wall clock time, the high order longword is incremented by one and the low order longword reset to zero. For subsequent modifications within the same second of wall clock time, the low order longword is incremented. If the low order longword wraps around to zero, the high order longword is incremented again. Since the high order longword only increments once per second and the inode is pushed to disk frequently during file modification, this implies 0 \(<= Current-Disk \(<= 5. When the inode is read in from disk, 10 is added to the high order longword, which ensures that the quadword is greater than any value it could have had before a crash. This introduces apparent modifications every time the inode falls out of the LRU inode cache, but this should only reduce the client caching performance by a (hopefully) small margin. .sh 2 "Crash Recovery and other Failure Scenarios" .pp The server must maintain the state of all the current leases held by clients. The nice thing about short term leases is that maximum_lease_term seconds after the server stops issuing leases, there are no current leases left. As such, server crash recovery does not require any state recovery. After rebooting, the server refuses to service any RPCs except for writes until write_slack seconds after the last lease would have expired\**. .(f \** The last lease expiry time may be safely estimated as "boottime+maximum_lease_term+clock_skew" for machines that cannot store it in nonvolatile RAM. .)f By then, the server would not have any outstanding leases to recover the state of and the clients have had at least write_slack seconds to push dirty writes to the server and get the server sync'd up to date. After this, the server simply services requests in a manner similar to NFS. In an effort to minimize the effect of "recovery storms" [Baker91], the server replies try_again_later to the RPCs it is not yet ready to service. .pp After a client crashes, the server may have to wait for a lease to timeout before servicing a request if write sharing of a file with a cachable lease on the client is about to occur. As for the client, it simply starts up getting any leases it now needs. Any outstanding leases for that client on the server prior to the crash will either be renewed or expire via timeout. .pp Certain network partitioning failures are more problematic. If a client to server network connection is severed just before a write caching lease expires, the client cannot push the dirty writes to the server. After the lease expires on the server, the server permits other clients to access the file with the potential of getting stale data. Unfortunately I believe this failure scenario is intrinsic in any delay write caching scheme unless the server is required to wait forever for a client to regain contact\**. .(f \** Gray and Cheriton avoid this problem by using a write through policy. .)f Since the write caching lease has expired on the client, it will sync up with the server as soon as the network connection has been re-established. .pp There is another failure condition that can occur when the server is congested. The worst case scenario would have the client pushing dirty writes to the server but a large request queue on the server delays these writes for more than write_slack seconds. It is hoped that a congestion control scheme using the try_again_later RPC reply after booting combined with the following lease termination rule for write caching leases can minimize the risk of this occurrence. A write caching lease is only terminated on the server when there are have been no writes to the file and the server has not been overloaded during the previous write_slack seconds. The server has not been overloaded is approximated by a test for sleeping nfsd(s) at the end of the write_slack period. .sh 2 "Server Disk Full" .pp There is a serious unresolved problem for delayed write caching with respect to server disk space allocation. When the disk on the file server is full, delayed write RPCs can fail due to "out of space". For NFS, this occurrence results in an error return from the close system call on the file, since the dirty blocks are pushed on close. Processes writing important files can check for this error return to ensure that the file was written successfully. For NQNFS, the dirty blocks are not pushed on close and as such the client may not attempt the write RPC until after the process has done the close which implies no error return from the close. For the current prototype, the only solution is to modify programs writing important file(s) to call fsync and check for an error return from it instead of close. .sh 2 "Protocol Details" .pp The protocol specification is identical to that of NFS [Sun89] except for the following changes. .ip \(bu RPC Information .(l Program Number 300105 Version Number 1 .)l .ip \(bu Readdir_and_Lookup RPC .(l struct readdirlookargs { fhandle file; nfscookie cookie; unsigned count; unsigned duration; }; struct entry { unsigned cachable; unsigned duration; modifyrev rev; fhandle entry_fh; nqnfs_fattr entry_attrib; unsigned fileid; filename name; nfscookie cookie; entry *nextentry; }; union readdirlookres switch (stat status) { case NFS_OK: struct { entry *entries; bool eof; } readdirlookok; default: void; }; readdirlookres NQNFSPROC_READDIRLOOK(readdirlookargs) = 18; .)l Reads entries in a directory in a manner analogous to the NFSPROC_READDIR RPC in NFS, but returns the file handle and attributes of each entry as well. This allows the attribute and lookup caches to be primed. .ip \(bu Get Lease RPC .(l struct getleaseargs { fhandle file; cachetype readwrite; unsigned duration; }; union getleaseres switch (stat status) { case NFS_OK: bool cachable; unsigned duration; modifyrev rev; nqnfs_fattr attributes; default: void; }; getleaseres NQNFSPROC_GETLEASE(getleaseargs) = 19; .)l Gets a lease for "file" valid for "duration" seconds from when the lease was issued on the server\**. .(f \** To be safe, the client may only assume that the lease is valid for ``duration'' seconds from when the RPC request was sent to the server. .)f The lease permits client caching if "cachable" is true. The modify revision level and attributes for the file are also returned. .ip \(bu Eviction Message .(l void NQNFSPROC_EVICTED (fhandle) = 21; .)l This message is sent from the server to the client. When the client receives the message, it should flush data associated with the file represented by "fhandle" from its caches and then send the Vacated Message back to the server. Flushing includes pushing any dirty writes via. write RPCs. .ip \(bu Vacated Message .(l void NQNFSPROC_VACATED (fhandle) = 20; .)l This message is sent from the client to the server in response to the Eviction Message. See above. .ip \(bu Access RPC .(l struct accessargs { fhandle file; bool read_access; bool write_access; bool exec_access; }; stat NQNFSPROC_ACCESS(accessargs) = 22; .)l The access RPC does permission checking on the server for the given type of access required by the client for the file. Use of this RPC avoids accessibility problems caused by client->server uid mapping. .ip \(bu Piggybacked Get Lease Request .pp The piggybacked get lease request is functionally equivalent to the Get Lease RPC except that is attached to one of the other NQNFS RPC requests as follows. A getleaserequest is prepended to all of the request arguments for NQNFS and a getleaserequestres is inserted in all NFS result structures just after the "stat" field only if "stat == NFS_OK". .(l union getleaserequest switch (cachetype type) { case NQLREAD: case NQLWRITE: unsigned duration; default: void; }; union getleaserequestres switch (cachetype type) { case NQLREAD: case NQLWRITE: bool cachable; unsigned duration; modifyrev rev; default: void; }; .)l The get lease request applies to the file that the attached RPC operates on and the file attributes remain in the same location as for the NFS RPC reply structure. .ip \(bu Three additional "stat" values .pp Three additional values have been added to the enumerated type "stat". .(l NQNFS_EXPIRED=500 NQNFS_TRYLATER=501 NQNFS_AUTHERR=502 .)l The "expired" value indicates that a lease has expired. The "try later" value is returned by the server when it wishes the client to retry the RPC request after a short delay. It is used during crash recovery (Section 2) and may also be useful for server congestion control. The "authetication error" value is returned for kerberized mount points to indicate that there is no cached authentication mapping and a Kerberos ticket for the principal is required. .sh 2 "Data Types" .ip \(bu cachetype .(l enum cachetype { NQLNONE = 0, NQLREAD = 1, NQLWRITE = 2 }; .)l Type of lease requested. NQLNONE is used to indicate no piggybacked lease request. .ip \(bu modifyrev .(l typedef unsigned hyper modifyrev; .)l The "modifyrev" is an unsigned quadword integer value that is never zero and increases every time the corresponding file is modified on the server. .ip \(bu nqnfs_time .(l struct nqnfs_time { unsigned seconds; unsigned nano_seconds; }; .)l For NQNFS times are handled at nano second resolution instead of micro second resolution for NFS. .ip \(bu nqnfs_fattr .(l struct nqnfs_fattr { ftype type; unsigned mode; unsigned nlink; unsigned uid; unsigned gid; unsigned hyper size; unsigned blocksize; unsigned rdev; unsigned hyper bytes; unsigned fsid; unsigned fileid; nqnfs_time atime; nqnfs_time mtime; nqnfs_time ctime; unsigned flags; unsigned generation; modifyrev rev; }; .)l The nqnfs_fattr structure is modified from the NFS fattr so that it stores the file size as a 64bit quantity and the storage occupied as a 64bit number of bytes. It also has fields added for the 4.4BSD va_flags and va_gen fields as well as the file's modify rev level. .ip \(bu nqnfs_sattr .(l struct nqnfs_sattr { unsigned mode; unsigned uid; unsigned gid; unsigned hyper size; nqnfs_time atime; nqnfs_time mtime; unsigned flags; unsigned rdev; }; .)l The nqnfs_sattr structure is modified from the NFS sattr structure in the same manner as fattr. .lp The arguments to several of the NFS RPCs have been modified as well. Mostly, these are minor changes to use 64bit file offsets or similar. The modified argument structures follow. .ip \(bu Lookup RPC .(l struct lookup_diropargs { unsigned duration; fhandle dir; filename name; }; union lookup_diropres switch (stat status) { case NFS_OK: struct { union getleaserequestres lookup_lease; fhandle file; nqnfs_fattr attributes; } lookup_diropok; default: void; }; .)l The additional "duration" argument tells the server to get a lease for the name being looked up if it is non-zero and the lease is specified in "lookup_lease". .ip \(bu Read RPC .(l struct nqnfs_readargs { fhandle file; unsigned hyper offset; unsigned count; }; .)l .ip \(bu Write RPC .(l struct nqnfs_writeargs { fhandle file; unsigned hyper offset; bool append; nfsdata data; }; .)l The "append" argument is true for apeend only write operations. .ip \(bu Get Filesystem Attributes RPC .(l union nqnfs_statfsres (stat status) { case NFS_OK: struct { unsigned tsize; unsigned bsize; unsigned blocks; unsigned bfree; unsigned bavail; unsigned files; unsigned files_free; } info; default: void; }; .)l The "files" field is the number of files in the file system and the "files_free" is the number of additional files that can be created. .sh 1 "Summary" .pp The configuration and tuning of an NFS environment tends to be a bit of a mystic art, but hopefully this paper along with the man pages and other reading will be helpful. Good Luck.
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. 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.
@(#)2.t 8.1 (Berkeley) 6/8/93
$FreeBSD$
.sh 1 "Not Quite NFS, Crash Tolerant Cache Consistency for NFS" .pp Not Quite NFS (NQNFS) is an NFS like protocol designed to maintain full cache consistency between clients in a crash tolerant manner. It is an adaptation of the NFS protocol such that the server supports both NFS and NQNFS clients while maintaining full consistency between the server and NQNFS clients. This section borrows heavily from work done on Spritely-NFS [Srinivasan89], but uses Leases [Gray89] to avoid the need to recover server state information after a crash. The reader is strongly encouraged to read these references before trying to grasp the material presented here. .sh 2 "Overview" .pp The protocol maintains cache consistency by using a somewhat Sprite [Nelson88] like protocol, but is based on short term leases\** instead of hard state information about open files. .(f \** A lease is a ticket permitting an activity that is valid until some expiry time. .)f The basic principal is that the protocol will disable client caching of a file whenever that file is write shared\**. .(f \** Write sharing occurs when at least one client is modifying a file while other client(s) are reading the file. .)f Whenever a client wishes to cache data for a file it must hold a valid lease. There are three types of leases: read caching, write caching and non-caching. The latter type requires that all file operations be done synchronously with the server via. RPCs. A read caching lease allows for client data caching, but no file modifications may be done. A write caching lease allows for client caching of writes, but requires that all writes be pushed to the server when the lease expires. If a client has dirty buffers\** .(f \** Cached write data is not yet pushed (written) to the server. .)f when a write cache lease has almost expired, it will attempt to extend the lease but is required to push the dirty buffers if extension fails. A client gets leases by either doing a GetLease RPC or by piggybacking a GetLease Request onto another RPC. Piggybacking is supported for the frequent RPCs Getattr, Setattr, Lookup, Readlink, Read, Write and Readdir in an effort to minimize the number of GetLease RPCs required. All leases are at the granularity of a file, since all NFS RPCs operate on individual files and NFS has no intrinsic notion of a file hierarchy. Directories, symbolic links and file attributes may be read cached but are not write cached. The exception here is the attribute file_size, which is updated during cached writing on the client to reflect a growing file. .pp It is the server's responsibility to ensure that consistency is maintained among the NQNFS clients by disabling client caching whenever a server file operation would cause inconsistencies. The possibility of inconsistencies occurs whenever a client has a write caching lease and any other client, or local operations on the server, tries to access the file or when a modify operation is attempted on a file being read cached by client(s). At this time, the server sends an eviction notice to all clients holding the lease and then waits for lease termination. Lease termination occurs when a vacated the premises message has been received from all the clients that have signed the lease or when the lease expires via. timeout. The message pair eviction notice and vacated the premises roughly correspond to a Sprite server\(->client callback, but are not implemented as an actual RPC, to avoid the server waiting indefinitely for a reply from a dead client. .pp Server consistency checking can be viewed as issuing intrinsic leases for a file operation for the duration of the operation only. For example, the Create RPC will get an intrinsic write lease on the directory in which the file is being created, disabling client read caches for that directory. .pp By relegating this responsibility to the server, consistency between the server and NQNFS clients is maintained when NFS clients are modifying the file system as well.\** .(f \** The NFS clients will continue to be approximately consistent with the server. .)f .pp The leases are issued as time intervals to avoid the requirement of time of day clock synchronization. There are three important time constants known to the server. The maximum_lease_term sets an upper bound on lease duration. The clock_skew is added to all lease terms on the server to correct for differing clock speeds between the client and server and write_slack is the number of seconds the server is willing to wait for a client with an expired write caching lease to push dirty writes. .pp The server maintains a modify_revision number for each file. It is defined as an unsigned quadword integer that is never zero and that must increase whenever the corresponding file is modified on the server. It is used by the client to determine whether or not cached data for the file is stale. Generating this value is easier said than done. The current implementation uses the following technique, which is believed to be adequate. The high order longword is stored in the ufs inode and is initialized to one when an inode is first allocated. The low order longword is stored in main memory only and is initialized to zero when an inode is read in from disk. When the file is modified for the first time within a given second of wall clock time, the high order longword is incremented by one and the low order longword reset to zero. For subsequent modifications within the same second of wall clock time, the low order longword is incremented. If the low order longword wraps around to zero, the high order longword is incremented again. Since the high order longword only increments once per second and the inode is pushed to disk frequently during file modification, this implies 0 \(<= Current-Disk \(<= 5. When the inode is read in from disk, 10 is added to the high order longword, which ensures that the quadword is greater than any value it could have had before a crash. This introduces apparent modifications every time the inode falls out of the LRU inode cache, but this should only reduce the client caching performance by a (hopefully) small margin. .sh 2 "Crash Recovery and other Failure Scenarios" .pp The server must maintain the state of all the current leases held by clients. The nice thing about short term leases is that maximum_lease_term seconds after the server stops issuing leases, there are no current leases left. As such, server crash recovery does not require any state recovery. After rebooting, the server refuses to service any RPCs except for writes until write_slack seconds after the last lease would have expired\**. .(f \** The last lease expiry time may be safely estimated as "boottime+maximum_lease_term+clock_skew" for machines that cannot store it in nonvolatile RAM. .)f By then, the server would not have any outstanding leases to recover the state of and the clients have had at least write_slack seconds to push dirty writes to the server and get the server sync'd up to date. After this, the server simply services requests in a manner similar to NFS. In an effort to minimize the effect of "recovery storms" [Baker91], the server replies try_again_later to the RPCs it is not yet ready to service. .pp After a client crashes, the server may have to wait for a lease to timeout before servicing a request if write sharing of a file with a cachable lease on the client is about to occur. As for the client, it simply starts up getting any leases it now needs. Any outstanding leases for that client on the server prior to the crash will either be renewed or expire via timeout. .pp Certain network partitioning failures are more problematic. If a client to server network connection is severed just before a write caching lease expires, the client cannot push the dirty writes to the server. After the lease expires on the server, the server permits other clients to access the file with the potential of getting stale data. Unfortunately I believe this failure scenario is intrinsic in any delay write caching scheme unless the server is required to wait forever for a client to regain contact\**. .(f \** Gray and Cheriton avoid this problem by using a write through policy. .)f Since the write caching lease has expired on the client, it will sync up with the server as soon as the network connection has been re-established. .pp There is another failure condition that can occur when the server is congested. The worst case scenario would have the client pushing dirty writes to the server but a large request queue on the server delays these writes for more than write_slack seconds. It is hoped that a congestion control scheme using the try_again_later RPC reply after booting combined with the following lease termination rule for write caching leases can minimize the risk of this occurrence. A write caching lease is only terminated on the server when there are have been no writes to the file and the server has not been overloaded during the previous write_slack seconds. The server has not been overloaded is approximated by a test for sleeping nfsd(s) at the end of the write_slack period. .sh 2 "Server Disk Full" .pp There is a serious unresolved problem for delayed write caching with respect to server disk space allocation. When the disk on the file server is full, delayed write RPCs can fail due to "out of space". For NFS, this occurrence results in an error return from the close system call on the file, since the dirty blocks are pushed on close. Processes writing important files can check for this error return to ensure that the file was written successfully. For NQNFS, the dirty blocks are not pushed on close and as such the client may not attempt the write RPC until after the process has done the close which implies no error return from the close. For the current prototype, the only solution is to modify programs writing important file(s) to call fsync and check for an error return from it instead of close. .sh 2 "Protocol Details" .pp The protocol specification is identical to that of NFS [Sun89] except for the following changes. .ip \(bu RPC Information .(l Program Number 300105 Version Number 1 .)l .ip \(bu Readdir_and_Lookup RPC .(l struct readdirlookargs { fhandle file; nfscookie cookie; unsigned count; unsigned duration; }; struct entry { unsigned cachable; unsigned duration; modifyrev rev; fhandle entry_fh; nqnfs_fattr entry_attrib; unsigned fileid; filename name; nfscookie cookie; entry *nextentry; }; union readdirlookres switch (stat status) { case NFS_OK: struct { entry *entries; bool eof; } readdirlookok; default: void; }; readdirlookres NQNFSPROC_READDIRLOOK(readdirlookargs) = 18; .)l Reads entries in a directory in a manner analogous to the NFSPROC_READDIR RPC in NFS, but returns the file handle and attributes of each entry as well. This allows the attribute and lookup caches to be primed. .ip \(bu Get Lease RPC .(l struct getleaseargs { fhandle file; cachetype readwrite; unsigned duration; }; union getleaseres switch (stat status) { case NFS_OK: bool cachable; unsigned duration; modifyrev rev; nqnfs_fattr attributes; default: void; }; getleaseres NQNFSPROC_GETLEASE(getleaseargs) = 19; .)l Gets a lease for "file" valid for "duration" seconds from when the lease was issued on the server\**. .(f \** To be safe, the client may only assume that the lease is valid for ``duration'' seconds from when the RPC request was sent to the server. .)f The lease permits client caching if "cachable" is true. The modify revision level and attributes for the file are also returned. .ip \(bu Eviction Message .(l void NQNFSPROC_EVICTED (fhandle) = 21; .)l This message is sent from the server to the client. When the client receives the message, it should flush data associated with the file represented by "fhandle" from its caches and then send the Vacated Message back to the server. Flushing includes pushing any dirty writes via. write RPCs. .ip \(bu Vacated Message .(l void NQNFSPROC_VACATED (fhandle) = 20; .)l This message is sent from the client to the server in response to the Eviction Message. See above. .ip \(bu Access RPC .(l struct accessargs { fhandle file; bool read_access; bool write_access; bool exec_access; }; stat NQNFSPROC_ACCESS(accessargs) = 22; .)l The access RPC does permission checking on the server for the given type of access required by the client for the file. Use of this RPC avoids accessibility problems caused by client->server uid mapping. .ip \(bu Piggybacked Get Lease Request .pp The piggybacked get lease request is functionally equivalent to the Get Lease RPC except that is attached to one of the other NQNFS RPC requests as follows. A getleaserequest is prepended to all of the request arguments for NQNFS and a getleaserequestres is inserted in all NFS result structures just after the "stat" field only if "stat == NFS_OK". .(l union getleaserequest switch (cachetype type) { case NQLREAD: case NQLWRITE: unsigned duration; default: void; }; union getleaserequestres switch (cachetype type) { case NQLREAD: case NQLWRITE: bool cachable; unsigned duration; modifyrev rev; default: void; }; .)l The get lease request applies to the file that the attached RPC operates on and the file attributes remain in the same location as for the NFS RPC reply structure. .ip \(bu Three additional "stat" values .pp Three additional values have been added to the enumerated type "stat". .(l NQNFS_EXPIRED=500 NQNFS_TRYLATER=501 NQNFS_AUTHERR=502 .)l The "expired" value indicates that a lease has expired. The "try later" value is returned by the server when it wishes the client to retry the RPC request after a short delay. It is used during crash recovery (Section 2) and may also be useful for server congestion control. The "authetication error" value is returned for kerberized mount points to indicate that there is no cached authentication mapping and a Kerberos ticket for the principal is required. .sh 2 "Data Types" .ip \(bu cachetype .(l enum cachetype { NQLNONE = 0, NQLREAD = 1, NQLWRITE = 2 }; .)l Type of lease requested. NQLNONE is used to indicate no piggybacked lease request. .ip \(bu modifyrev .(l typedef unsigned hyper modifyrev; .)l The "modifyrev" is an unsigned quadword integer value that is never zero and increases every time the corresponding file is modified on the server. .ip \(bu nqnfs_time .(l struct nqnfs_time { unsigned seconds; unsigned nano_seconds; }; .)l For NQNFS times are handled at nano second resolution instead of micro second resolution for NFS. .ip \(bu nqnfs_fattr .(l struct nqnfs_fattr { ftype type; unsigned mode; unsigned nlink; unsigned uid; unsigned gid; unsigned hyper size; unsigned blocksize; unsigned rdev; unsigned hyper bytes; unsigned fsid; unsigned fileid; nqnfs_time atime; nqnfs_time mtime; nqnfs_time ctime; unsigned flags; unsigned generation; modifyrev rev; }; .)l The nqnfs_fattr structure is modified from the NFS fattr so that it stores the file size as a 64bit quantity and the storage occupied as a 64bit number of bytes. It also has fields added for the 4.4BSD va_flags and va_gen fields as well as the file's modify rev level. .ip \(bu nqnfs_sattr .(l struct nqnfs_sattr { unsigned mode; unsigned uid; unsigned gid; unsigned hyper size; nqnfs_time atime; nqnfs_time mtime; unsigned flags; unsigned rdev; }; .)l The nqnfs_sattr structure is modified from the NFS sattr structure in the same manner as fattr. .lp The arguments to several of the NFS RPCs have been modified as well. Mostly, these are minor changes to use 64bit file offsets or similar. The modified argument structures follow. .ip \(bu Lookup RPC .(l struct lookup_diropargs { unsigned duration; fhandle dir; filename name; }; union lookup_diropres switch (stat status) { case NFS_OK: struct { union getleaserequestres lookup_lease; fhandle file; nqnfs_fattr attributes; } lookup_diropok; default: void; }; .)l The additional "duration" argument tells the server to get a lease for the name being looked up if it is non-zero and the lease is specified in "lookup_lease". .ip \(bu Read RPC .(l struct nqnfs_readargs { fhandle file; unsigned hyper offset; unsigned count; }; .)l .ip \(bu Write RPC .(l struct nqnfs_writeargs { fhandle file; unsigned hyper offset; bool append; nfsdata data; }; .)l The "append" argument is true for apeend only write operations. .ip \(bu Get Filesystem Attributes RPC .(l union nqnfs_statfsres (stat status) { case NFS_OK: struct { unsigned tsize; unsigned bsize; unsigned blocks; unsigned bfree; unsigned bavail; unsigned files; unsigned files_free; } info; default: void; }; .)l The "files" field is the number of files in the file system and the "files_free" is the number of additional files that can be created. .sh 1 "Summary" .pp The configuration and tuning of an NFS environment tends to be a bit of a mystic art, but hopefully this paper along with the man pages and other reading will be helpful. Good Luck.