Document -o writeback_cache in manpage

[waldner]

After upgrading to 3.2.0, a program that was writing files in an sshfs-mounted directory started reporting errors. With strace, I see that what fails is the following operation:

open("/sshfs/mounted/dir/2017-08-16.184559+0200CEST.txt", O_WRONLY|O_CREAT|O_APPEND, 0666) = -1 EINVAL (Invalid argument)

I'm not sure what has changed with 3.2, surely I didn't do anything different. The directory is mounted with the following line in fstab:

user@host:/remote/dir /sshfs/mounted/dir fuse.sshfs noauto,x-systemd.automount,_netdev,user,idmap=user,reconnect,transform_symlinks,identityfile=/path/to/id_ed25519,addressfamily=inet,port=45678,allow_other,default_permissions,uid=1000,gid=1000 0 0

Also, I can create files under the directory in question (with touch, redirections etc) from the shell, as the same user the program runs as.

[Nikratio]

Probably the problem is with O_APPEND. From the manpage:

CAVEATS / WORKAROUNDS
[...]
   O_APPEND
       When  writeback  caching is enabled, SSHFS cannot reliably support the O_APPEND open
       flag and thus signals an error on open.  To enable support for  unreliable  O_APPEND
       (which  may  overwrite  data if the file changes on the server at a bad time), mount
       the file system with -o unreliable_append.

Does that help?

[waldner]

Well, I can't change the application behavior, so I'll try with the unreliable_append flag. Thanks

[waldner]

With unreliable_append it does work. Thanks!

[Nikratio]

Alternatively, you could also disable writeback cache.

[waldner]

Which man page are you referring to? I don't see the section you mentioned earlier in the man page on my system, and not even here: https://github.com/libfuse/sshfs/blob/master/sshfs.1.in. It looks like it's the man page for SSHFS version 2.0? Also, on my system running "sshfs --version" produces

SSHFS version 3.1.0
FUSE library version 3.1.1
using FUSE kernel interface version 7.26
fusermount3 version: 3.1.1

but the package manager tells me that I have 3.2.0 installed (and the fact that unreliable_append works seems to confirm it). Is the wrong version number a distro issue (Arch Linux)?

[Nikratio]

The one that so far only lived in my local clone and that I just pushed to GitHub of course! What else?

(Sorry!)

[Nikratio]

https://github.com/libfuse/sshfs/blob/master/sshfs.rst

[waldner]

Well, I still don't see how to disable writeback cache from that man page :-)

[pounard]

@Nikratio Searching this repository I could find this: https://github.com/libfuse/sshfs/search?utf8=%E2%9C%93&q=writeback_cache&type=

From what I see, you should set -o writeback_cache=no I guess.

I stumbled upon the same bug after upgrade, so I'll try it right now :)

[plroman]

Same problem when using git with sshfs 3.2, I don't get these errors with 3.0:

$ git pull
error: cannot open .git/FETCH_HEAD: Invalid argument
$ git commit -m 'should have used svn'
fatal: cannot update the ref 'HEAD': unable to append to '.git/logs/HEAD': Invalid argument

$ strace git commit -m 'or maybe cvs'
[...]
open(".git/logs/HEAD", O_WRONLY|O_CREAT|O_APPEND, 0666) = -1 EINVAL (Invalid argument)
[...]

It works with -o writeback_cache=no (or -o unreliable_append). Thank you for that quick fix.

[pounard]

I still have a lot of random failures with git (at least once or twice a day, hence the "random") which I didn't have with versions <= 3 (even after I set the writeback_cache option)

[Nikratio]

Looking at issue #88, we should make sure that the documentation explains that behavior.