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1Reexporting NFS filesystems2===========================3 4Overview5--------6 7It is possible to reexport an NFS filesystem over NFS. However, this8feature comes with a number of limitations. Before trying it, we9recommend some careful research to determine whether it will work for10your purposes.11 12A discussion of current known limitations follows.13 14"fsid=" required, crossmnt broken15---------------------------------16 17We require the "fsid=" export option on any reexport of an NFS18filesystem. You can use "uuidgen -r" to generate a unique argument.19 20The "crossmnt" export does not propagate "fsid=", so it will not allow21traversing into further nfs filesystems; if you wish to export nfs22filesystems mounted under the exported filesystem, you'll need to export23them explicitly, assigning each its own unique "fsid= option.24 25Reboot recovery26---------------27 28The NFS protocol's normal reboot recovery mechanisms don't work for the29case when the reexport server reboots. Clients will lose any locks30they held before the reboot, and further IO will result in errors.31Closing and reopening files should clear the errors.32 33Filehandle limits34-----------------35 36If the original server uses an X byte filehandle for a given object, the37reexport server's filehandle for the reexported object will be X+2238bytes, rounded up to the nearest multiple of four bytes.39 40The result must fit into the RFC-mandated filehandle size limits:41 42+-------+-----------+43| NFSv2 | 32 bytes |44+-------+-----------+45| NFSv3 | 64 bytes |46+-------+-----------+47| NFSv4 | 128 bytes |48+-------+-----------+49 50So, for example, you will only be able to reexport a filesystem over51NFSv2 if the original server gives you filehandles that fit in 1052bytes--which is unlikely.53 54In general there's no way to know the maximum filehandle size given out55by an NFS server without asking the server vendor.56 57But the following table gives a few examples. The first column is the58typical length of the filehandle from a Linux server exporting the given59filesystem, the second is the length after that nfs export is reexported60by another Linux host:61 62+--------+-------------------+----------------+63| | filehandle length | after reexport |64+========+===================+================+65| ext4: | 28 bytes | 52 bytes |66+--------+-------------------+----------------+67| xfs: | 32 bytes | 56 bytes |68+--------+-------------------+----------------+69| btrfs: | 40 bytes | 64 bytes |70+--------+-------------------+----------------+71 72All will therefore fit in an NFSv3 or NFSv4 filehandle after reexport,73but none are reexportable over NFSv2.74 75Linux server filehandles are a bit more complicated than this, though;76for example:77 78 - The (non-default) "subtreecheck" export option generally79 requires another 4 to 8 bytes in the filehandle.80 - If you export a subdirectory of a filesystem (instead of81 exporting the filesystem root), that also usually adds 4 to 882 bytes.83 - If you export over NFSv2, knfsd usually uses a shorter84 filesystem identifier that saves 8 bytes.85 - The root directory of an export uses a filehandle that is86 shorter.87 88As you can see, the 128-byte NFSv4 filehandle is large enough that89you're unlikely to have trouble using NFSv4 to reexport any filesystem90exported from a Linux server. In general, if the original server is91something that also supports NFSv3, you're *probably* OK. Re-exporting92over NFSv3 may be dicier, and reexporting over NFSv2 will probably93never work.94 95For more details of Linux filehandle structure, the best reference is96the source code and comments; see in particular:97 98 - include/linux/exportfs.h:enum fid_type99 - include/uapi/linux/nfsd/nfsfh.h:struct nfs_fhbase_new100 - fs/nfsd/nfsfh.c:set_version_and_fsid_type101 - fs/nfs/export.c:nfs_encode_fh102 103Open DENY bits ignored104----------------------105 106NFS since NFSv4 supports ALLOW and DENY bits taken from Windows, which107allow you, for example, to open a file in a mode which forbids other108read opens or write opens. The Linux client doesn't use them, and the109server's support has always been incomplete: they are enforced only110against other NFS users, not against processes accessing the exported111filesystem locally. A reexport server will also not pass them along to112the original server, so they will not be enforced between clients of113different reexport servers.114