From dc451bbc6f54a2c7bacef7ec445718709071b61c Mon Sep 17 00:00:00 2001 From: J. Bruce Fields Date: Fri, 10 Sep 2021 14:42:54 -0400 Subject: nfs: reexport documentation We've supported reexport for a while but documentation is limited. This is mainly a simplified version of the text I wrote for the linux-nfs wiki at https://wiki.linux-nfs.org/wiki/index.php/NFS_re-export. Signed-off-by: J. Bruce Fields --- Documentation/filesystems/nfs/index.rst | 1 + Documentation/filesystems/nfs/reexport.rst | 113 +++++++++++++++++++++++++++++ 2 files changed, 114 insertions(+) create mode 100644 Documentation/filesystems/nfs/reexport.rst (limited to 'Documentation') diff --git a/Documentation/filesystems/nfs/index.rst b/Documentation/filesystems/nfs/index.rst index 65805624e39b..288d8ddb2bc6 100644 --- a/Documentation/filesystems/nfs/index.rst +++ b/Documentation/filesystems/nfs/index.rst @@ -11,3 +11,4 @@ NFS rpc-server-gss nfs41-server knfsd-stats + reexport diff --git a/Documentation/filesystems/nfs/reexport.rst b/Documentation/filesystems/nfs/reexport.rst new file mode 100644 index 000000000000..ff9ae4a46530 --- /dev/null +++ b/Documentation/filesystems/nfs/reexport.rst @@ -0,0 +1,113 @@ +Reexporting NFS filesystems +=========================== + +Overview +-------- + +It is possible to reexport an NFS filesystem over NFS. However, this +feature comes with a number of limitations. Before trying it, we +recommend some careful research to determine whether it will work for +your purposes. + +A discussion of current known limitations follows. + +"fsid=" required, crossmnt broken +--------------------------------- + +We require the "fsid=" export option on any reexport of an NFS +filesystem. You can use "uuidgen -r" to generate a unique argument. + +The "crossmnt" export does not propagate "fsid=", so it will not allow +traversing into further nfs filesystems; if you wish to export nfs +filesystems mounted under the exported filesystem, you'll need to export +them explicitly, assigning each its own unique "fsid= option. + +Reboot recovery +--------------- + +The NFS protocol's normal reboot recovery mechanisms don't work for the +case when the reexport server reboots. Clients will lose any locks +they held before the reboot, and further IO will result in errors. +Closing and reopening files should clear the errors. + +Filehandle limits +----------------- + +If the original server uses an X byte filehandle for a given object, the +reexport server's filehandle for the reexported object will be X+22 +bytes, rounded up to the nearest multiple of four bytes. + +The result must fit into the RFC-mandated filehandle size limits: + ++-------+-----------+ +| NFSv2 | 32 bytes | ++-------+-----------+ +| NFSv3 | 64 bytes | ++-------+-----------+ +| NFSv4 | 128 bytes | ++-------+-----------+ + +So, for example, you will only be able to reexport a filesystem over +NFSv2 if the original server gives you filehandles that fit in 10 +bytes--which is unlikely. + +In general there's no way to know the maximum filehandle size given out +by an NFS server without asking the server vendor. + +But the following table gives a few examples. The first column is the +typical length of the filehandle from a Linux server exporting the given +filesystem, the second is the length after that nfs export is reexported +by another Linux host: + ++--------+-------------------+----------------+ +| | filehandle length | after reexport | ++========+===================+================+ +| ext4: | 28 bytes | 52 bytes | ++--------+-------------------+----------------+ +| xfs: | 32 bytes | 56 bytes | ++--------+-------------------+----------------+ +| btrfs: | 40 bytes | 64 bytes | ++--------+-------------------+----------------+ + +All will therefore fit in an NFSv3 or NFSv4 filehandle after reexport, +but none are reexportable over NFSv2. + +Linux server filehandles are a bit more complicated than this, though; +for example: + + - The (non-default) "subtreecheck" export option generally + requires another 4 to 8 bytes in the filehandle. + - If you export a subdirectory of a filesystem (instead of + exporting the filesystem root), that also usually adds 4 to 8 + bytes. + - If you export over NFSv2, knfsd usually uses a shorter + filesystem identifier that saves 8 bytes. + - The root directory of an export uses a filehandle that is + shorter. + +As you can see, the 128-byte NFSv4 filehandle is large enough that +you're unlikely to have trouble using NFSv4 to reexport any filesystem +exported from a Linux server. In general, if the original server is +something that also supports NFSv3, you're *probably* OK. Re-exporting +over NFSv3 may be dicier, and reexporting over NFSv2 will probably +never work. + +For more details of Linux filehandle structure, the best reference is +the source code and comments; see in particular: + + - include/linux/exportfs.h:enum fid_type + - include/uapi/linux/nfsd/nfsfh.h:struct nfs_fhbase_new + - fs/nfsd/nfsfh.c:set_version_and_fsid_type + - fs/nfs/export.c:nfs_encode_fh + +Open DENY bits ignored +---------------------- + +NFS since NFSv4 supports ALLOW and DENY bits taken from Windows, which +allow you, for example, to open a file in a mode which forbids other +read opens or write opens. The Linux client doesn't use them, and the +server's support has always been incomplete: they are enforced only +against other NFS users, not against processes accessing the exported +filesystem locally. A reexport server will also not pass them along to +the original server, so they will not be enforced between clients of +different reexport servers. -- cgit v1.2.3 From 6d91929a6fa6b340241bf09ded8740cd439d4df3 Mon Sep 17 00:00:00 2001 From: J. Bruce Fields Date: Mon, 1 Nov 2021 15:17:53 -0400 Subject: nfsd: document server-to-server-copy parameters Signed-off-by: J. Bruce Fields --- Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'Documentation') diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 91ba391f9b32..14bc3f0b0149 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -3243,6 +3243,19 @@ driver. A non-zero value sets the minimum interval in seconds between layoutstats transmissions. + nfsd.inter_copy_offload_enable = + [NFSv4.2] When set to 1, the server will support + server-to-server copies for which this server is + the destination of the copy. + + nfsd.nfsd4_ssc_umount_timeout = + [NFSv4.2] When used as the destination of a + server-to-server copy, knfsd temporarily mounts + the source server. It caches the mount in case + it will be needed again, and discards it if not + used for the number of milliseconds specified by + this parameter. + nfsd.nfs4_disable_idmapping= [NFSv4] When set to the default of '1', the NFSv4 server will return only numeric uids and gids to @@ -3250,6 +3263,7 @@ and gids from such clients. This is intended to ease migration from NFSv2/v3. + nmi_backtrace.backtrace_idle [KNL] Dump stacks even of idle CPUs in response to an NMI stack-backtrace request. -- cgit v1.2.3