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+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.