[Linux-cachefs] cachefilesd refuses to cache NFS exports/mounts with fsid = 0

[David Härdeman]

Hi,

I've been trying to setup fscache for an NFS share by following various online guides but the client refused to enable the FSC for some reason (client = Debian Unstable, Linux 5.6.0; server = Debian Stable, Linux 4.19.0-8).

After a lot of trial and error, I discovered that the caching is enabled as long as the fsid is not zero.

Is this expected behavior and, if so, shouldn't it be documented somewhere?

Cheers,
David


Test summary
============

(root at server:/etc/exports.d)$ cat srv.exports 
# Note: /srv is a separate partition
/srv	<IPv6 ULA>::/64(sec=krb5:krb5i:krb5p,rw,async,crossmnt,no_subtree_check,root_squash)

(root at server:/etc/exports.d)$ exportfs -ra

(root at client:~)$ mount -t nfs4 server:/ /mnt/tmp -o sec=krb5,fsc

(root at client:~)$ cat /proc/fs/nfsfs/*
NV SERVER             PORT USE HOSTNAME
v4 <server IPv6 addr>  801   1 server
NV SERVER             PORT DEV          FSID                              FSC
v4 <server IPv6 addr>  801 0:70         0:0                               no 

(user at client:~)$ cat /mnt/tmp/srv/somedir/somefile.txt &> /dev/null

(root at client:~)$ cat /proc/fs/nfsfs/*
NV SERVER             PORT USE HOSTNAME
v4 <server IPv6 addr>  801   1 server
NV SERVER             PORT DEV          FSID                              FSC
v4 <server IPv6 addr>  801 0:70         0:0                               no 
v4 <server IPv6 addr>  801 0:73         7a26f4d97cd54396:9fffb68912b8d546 yes

(root at client:~)$ umount /mnt/tmp

(root at client:~)$ mount -t nfs4 server:/srv /mnt/tmp -o sec=krb5,fsc

(root at client:~)$ cat /proc/fs/nfsfs/*
NV SERVER             PORT USE HOSTNAME
v4 <server IPv6 addr>  801   1 server
NV SERVER             PORT DEV          FSID                              FSC
v4 <server IPv6 addr>  801 0:73         7a26f4d97cd54396:9fffb68912b8d546 yes

(root at client:~)$ umount /mnt/tmp

(root at server:/etc/exports.d)$ vi srv.exports 

(root at server:/etc/exports.d)$ cat srv.exports 
# Note: /srv is a separate partition
/srv	<IPv6 ULA>::/64(sec=krb5:krb5i:krb5p,rw,async,crossmnt,no_subtree_check,root_squash,fsid=0)

(root at server:/etc/exports.d)$ exportfs -ra

(root at client:~)$ mount -t nfs4 server:/ /mnt/tmp -o sec=krb5,fsc

(root at client:~)$ cat /proc/fs/nfsfs/*
NV SERVER             PORT USE HOSTNAME
v4 <server IPv6 addr>  801   1 server
NV SERVER             PORT DEV          FSID                              FSC
v4 <server IPv6 addr>  801 0:70         0:0                               no 

(user at client:~)$ cat /mnt/tmp/somedir/somefile.txt &> /dev/null

(root at client:~)$ cat /proc/fs/nfsfs/*
NV SERVER             PORT USE HOSTNAME
v4 <server IPv6 addr>  801   1 server
NV SERVER             PORT DEV          FSID                              FSC
v4 <server IPv6 addr>  801 0:70         0:0                               no