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Add export-support option as possible workaround for NFS EIO issues

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Antonio SJ Musumeci 2024-02-18 17:58:53 -06:00
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191
README.md
View File

@ -180,6 +180,9 @@ These options are the same regardless of whether you use them with the
link to always use the non-path preserving behavior. This means
files, when renamed or linked, will stay on the same
filesystem. (default: false)
* **export-support=BOOL**: Sets a low-level FUSE feature intended to
indicate the filesystem can support being exported via
NFS. (default: true)
* **security_capability=BOOL**: If false return ENOATTR when xattr
security.capability is queried. (default: true)
* **xattr=passthrough|noattr|nosys**: Runtime control of
@ -415,7 +418,7 @@ or file is to be selected or something changes out of band it becomes
unclear what value should be used. Most software does not to care what
the values are but those that do often break if a value changes
unexpectedly. The tool `find` will abort a directory walk if it sees a
directory inode change. NFS will return stale handle errors if the
directory inode change. NFS can return stale handle errors if the
inode changes out of band. File dedup tools will usually leverage
device ids and inodes as a shortcut in searching for duplicate files
and would resort to full file comparisons should it find different
@ -674,6 +677,27 @@ it will only perform the hack when the path in question includes
is empty is opened for writing.
### export-support
In theory this flag should not be exposed to the end user. It is a
low-level FUSE flag which indicates whether or not the kernel can send
certain kinds of messages to it for the purposes of using with
NFS. mergerfs does support these messages but due to a possible bug in
the Linux kernel this option needs to be configurable. As it turns out
at least certain versions of NFS can still work when disabled and no
longer triggers the bug.
While it may be safe to default this setting to `false` in an
abundance of caution it is being left enabled by default.
Given that this flag is set when the FUSE connection is first
initiated it is not possible to change during run time.
See [Kernel Issues &
Bugs](https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs)
for more details.
# FUNCTIONS, CATEGORIES and POLICIES
The POSIX filesystem API is made up of a number of
@ -1845,13 +1869,9 @@ on page caching.
#### NFS clients returning ESTALE / Stale file handle
NFS does not like out of band changes. That is especially true of
inode values.
Be sure to use the following options:
* noforget
* inodecalc=path-hash
NFS generally does not like out of band changes. Take a look at the
section on NFS in the [#remote-filesystems](#remote-filesystems) for
more details.
#### rtorrent fails with ENODEV (No such device)
@ -2001,6 +2021,123 @@ containers or use a standard shared user & groups technology like NIS
or LDAP.
# Remote Filesystems
Many users ask about compatibility with remote filesystems. This
section is to describe any known issues or quirks when using mergerfs
with common remote filesystems.
Keep in mind that, like with caching, it is not a good idea to change
the contents of the remote filesystem
[out-of-band](https://en.wikipedia.org/wiki/Out-of-band). Meaning that
you really shouldn't change the contents of the underlying
filesystems or mergerfs on the server hosting the remote
filesystem. Doing so can lead to weird behavior, inconsistency,
errors, and even data corruption should multiple programs try to write
or read the same data at the same time. This isn't to say you can't do
it or that data corruption is likely but it *could* happen. It is
better to always use the remote filesystem. Even on the machine
serving it.
## NFS
[NFS](https://en.wikipedia.org/wiki/Network_File_System) is a common
remote filesystem on Unix/POSIX systems. Due to how NFS works there
are some settings which need to be set in order for mergerfs to work
with it.
It should be noted that NFS and FUSE (the technology mergerfs uses)
do not work perfectly with one another due to certain design
choices in FUSE (and mergerfs.) Due to these issues it is
generally recommended to use SMB when possible till situations
change. That said issues should still be reported. NFS is not really
recommended but it isn't unsupported.
When exporting mergerfs via NFS ensure to set the following config:
* noforget
* inodecalc=path-hash
* export-support=false (only available in v2.40.0 and above)
`noforget` is needed because NFS uses the `name_to_handle_at` and
`open_by_handle_at` functions which allow a program to keep a
reference to a file without technically having it open in the typical
sense. The problem is that FUSE has no way to know that NFS has a
handle that it will later use to open the file again. As a result it
is possible for the kernel to tell mergerfs to forget about the node
and should NFS ever ask for that node's details in the future it would
have nothing to respond with. Keeping nodes around forever is not
ideal but at the moment the only way to manage the situation.
`inodecalc=path-hash` is needed because NFS is sensitive to
out-of-band changes. FUSE doesn't care if a file's inode value changes
but NFS, being stateful, does. So if you used the default inode
calculation algorithm then it is possible that if you changed a file
or updated a directory the file mergerfs will use will be on a
different branch and therefore the inode would change. This isn't an
ideal solution and others are being considered but it works for most
situations. It might be possible to leave the default value of
`hybrid-hash` if `export-support=false` but it hasn't been fully
tested yet.
`export-support=false` is a low-level FUSE option that is needed as a
workaround to a Linux kernel bug. Technically this option should be
set to `true` when expecting to use mergerfs with NFS but it turns out
that at least some versions of NFS work when not enabled. At the
moment it is unclear if this has any negative side effects but when
disabled NFS appears to work alright and the kernel no longer sends
invalid requests. When the issue is resolved in the kernel these docs
will be updated with appropriate details.
Also see [Kernel Issues &
Bugs](https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs)
for more details.
## SMB / CIFS
[SMB](https://en.wikipedia.org/wiki/Server_Message_Block) is a
protocol most used by Microsoft Windows systems to share file shares,
printers, etc. However, due to the popularity for Windows, it is also
supported on many other platforms including Linux. The most popular
way of supporting SMB on Linux is via the software Samba.
[Samba](https://en.wikipedia.org/wiki/Samba_(software)), and other
ways of serving Linux filesystems, via SMB should work fine with
mergerfs. The services do not tend to use the same technologies which
NFS uses and therefore don't have the same issues. There should not be
an special settings required to use mergerfs with Samba. However,
[CIFSD](https://en.wikipedia.org/wiki/CIFSD) and other programs have
not been extensively tested. If you use mergerfs with CIFSD or other
SMB servers please submit your experiences so these docs can be
updated.
## SSHFS
[SSHFS](https://en.wikipedia.org/wiki/SSHFS) is a FUSE filesystem
leveraging SSH as the connection and transport layer. While often
simpler to setup when compared to NFS or Samba the performance can be
lacking and the project is very much in maintenance mode.
There are no known issues using sshfs with mergerfs. You may want to
use the following arguments to improve performance but your millage
may vary.
* `-o Ciphers=arcfour`
* `-o Compression=no`
More info can be found
[here](https://ideatrash.net/2016/08/odds-and-ends-optimizing-sshfs-moving.html).
## Other
There are other remote filesystems but none popularly used to serve
mergerfs. If you use something not listed above feel free to reach out
and I will add it to the list.
# FAQ
#### How well does mergerfs scale? Is it "production ready?"
@ -2237,8 +2374,8 @@ values are correct and that directories have their executable bit
set. A common mistake by users new to Linux is to `chmod -R 644` when
they should have `chmod -R u=rwX,go=rX`.
If using a network filesystem such as NFS, SMB, CIFS (Samba) be sure
to pay close attention to anything regarding permissioning and
If using a network filesystem such as NFS or SMB (Samba) be sure to
pay close attention to anything regarding permissioning and
users. Root squashing and user translation for instance has bitten a
few mergerfs users. Some of these also affect the use of mergerfs from
container platforms such as Docker.
@ -2360,37 +2497,13 @@ it won't show up.
You can remove the reserve by running: `tune2fs -m 0 <device>`
#### Can mergerfs mounts be exported over NFS?
Yes, however if you do anything which may changes files out of band
(including for example using the `newest` policy) it will result in
"stale file handle" errors unless properly setup.
Be sure to use the following options:
* noforget
* inodecalc=path-hash
#### Can mergerfs mounts be exported over Samba / SMB?
Yes. While some users have reported problems it appears to always be
related to how Samba is setup in relation to permissions.
#### Can mergerfs mounts be used over SSHFS?
Yes.
#### I notice massive slowdowns of writes when enabling cache.files.
When file caching is enabled in any form (`cache.files!=off` or
`direct_io=false`) it will issue `getxattr` requests for
`security.capability` prior to *every single write*. This will usually
result in a performance degradation, especially when using a network
filesystem (such as NFS or CIFS/SMB/Samba.) Unfortunately at this
moment the kernel is not caching the response.
When file caching is enabled in any form (`cache.files!=off`) it will
issue `getxattr` requests for `security.capability` prior to *every
single write*. This will usually result in a performance degradation,
especially when using a network filesystem (such as NFS or SMB.)
Unfortunately at this moment the kernel is not caching the response.
To work around this situation mergerfs offers a few solutions.

1
libfuse/lib/fuse.c
View File

@ -1523,7 +1523,6 @@ fuse_lib_init(void *data,
memset(c,0,sizeof(*c));
c->ctx.fuse = f;
conn->want |= FUSE_CAP_EXPORT_SUPPORT;
f->fs->op.init(conn);
}

191
man/mergerfs.1
View File

@ -214,6 +214,10 @@ This means files, when renamed or linked, will stay on the same
filesystem.
(default: false)
.IP \[bu] 2
\f[B]export-support=BOOL\f[R]: Sets a low-level FUSE feature intended to
indicate the filesystem can support being exported via NFS.
(default: true)
.IP \[bu] 2
\f[B]security_capability=BOOL\f[R]: If false return ENOATTR when xattr
security.capability is queried.
(default: true)
@ -547,7 +551,7 @@ Most software does not to care what the values are but those that do
often break if a value changes unexpectedly.
The tool \f[C]find\f[R] will abort a directory walk if it sees a
directory inode change.
NFS will return stale handle errors if the inode changes out of band.
NFS can return stale handle errors if the inode changes out of band.
File dedup tools will usually leverage device ids and inodes as a
shortcut in searching for duplicate files and would resort to full file
comparisons should it find different inode values.
@ -837,6 +841,26 @@ If set to \f[C]git\f[R] it will only perform the hack when the path in
question includes \f[C]/.git/\f[R].
\f[C]all\f[R] will result it applying anytime a read-only file which is
empty is opened for writing.
.SS export-support
.PP
In theory this flag should not be exposed to the end user.
It is a low-level FUSE flag which indicates whether or not the kernel
can send certain kinds of messages to it for the purposes of using with
NFS.
mergerfs does support these messages but due to a possible bug in the
Linux kernel this option needs to be configurable.
As it turns out at least certain versions of NFS can still work when
disabled and no longer triggers the bug.
.PP
While it may be safe to default this setting to \f[C]false\f[R] in an
abundance of caution it is being left enabled by default.
.PP
Given that this flag is set when the FUSE connection is first initiated
it is not possible to change during run time.
.PP
See Kernel Issues &
Bugs (https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs)
for more details.
.SH FUNCTIONS, CATEGORIES and POLICIES
.PP
The POSIX filesystem API is made up of a number of functions.
@ -2375,14 +2399,9 @@ Use \f[C]cache.files=off\f[R] and/or \f[C]dropcacheonclose=true\f[R].
See the section on page caching.
.SS NFS clients returning ESTALE / Stale file handle
.PP
NFS does not like out of band changes.
That is especially true of inode values.
.PP
Be sure to use the following options:
.IP \[bu] 2
noforget
.IP \[bu] 2
inodecalc=path-hash
NFS generally does not like out of band changes.
Take a look at the section on NFS in the #remote-filesystems for more
details.
.SS rtorrent fails with ENODEV (No such device)
.PP
Be sure to set \f[C]cache.files=partial|full|auto-full|per-processe\f[R]
@ -2539,6 +2558,127 @@ There might be a hack to work around this (make mergerfs read the
would be limited to Linux and the /etc/group DB.
Preferably users would mount in the host group file into the containers
or use a standard shared user & groups technology like NIS or LDAP.
.SH Remote Filesystems
.PP
Many users ask about compatibility with remote filesystems.
This section is to describe any known issues or quirks when using
mergerfs with common remote filesystems.
.PP
Keep in mind that, like with caching, it is not a good idea to change
the contents of the remote filesystem
out-of-band (https://en.wikipedia.org/wiki/Out-of-band).
Meaning that you really shouldn\[cq]t change the contents of the
underlying filesystems or mergerfs on the server hosting the remote
filesystem.
Doing so can lead to weird behavior, inconsistency, errors, and even
data corruption should multiple programs try to write or read the same
data at the same time.
This isn\[cq]t to say you can\[cq]t do it or that data corruption is
likely but it \f[I]could\f[R] happen.
It is better to always use the remote filesystem.
Even on the machine serving it.
.SS NFS
.PP
NFS (https://en.wikipedia.org/wiki/Network_File_System) is a common
remote filesystem on Unix/POSIX systems.
Due to how NFS works there are some settings which need to be set in
order for mergerfs to work with it.
.PP
It should be noted that NFS and FUSE (the technology mergerfs uses) do
not work perfectly with one another due to certain design choices in
FUSE (and mergerfs.) Due to these issues it is generally recommended to
use SMB when possible till situations change.
That said issues should still be reported.
NFS is not really recommended but it isn\[cq]t unsupported.
.PP
When exporting mergerfs via NFS ensure to set the following config: *
noforget * inodecalc=path-hash * export-support=false (only available in
v2.40.0 and above)
.PP
\f[C]noforget\f[R] is needed because NFS uses the
\f[C]name_to_handle_at\f[R] and \f[C]open_by_handle_at\f[R] functions
which allow a program to keep a reference to a file without technically
having it open in the typical sense.
The problem is that FUSE has no way to know that NFS has a handle that
it will later use to open the file again.
As a result it is possible for the kernel to tell mergerfs to forget
about the node and should NFS ever ask for that node\[cq]s details in
the future it would have nothing to respond with.
Keeping nodes around forever is not ideal but at the moment the only way
to manage the situation.
.PP
\f[C]inodecalc=path-hash\f[R] is needed because NFS is sensitive to
out-of-band changes.
FUSE doesn\[cq]t care if a file\[cq]s inode value changes but NFS, being
stateful, does.
So if you used the default inode calculation algorithm then it is
possible that if you changed a file or updated a directory the file
mergerfs will use will be on a different branch and therefore the inode
would change.
This isn\[cq]t an ideal solution and others are being considered but it
works for most situations.
It might be possible to leave the default value of \f[C]hybrid-hash\f[R]
if \f[C]export-support=false\f[R] but it hasn\[cq]t been fully tested
yet.
.PP
\f[C]export-support=false\f[R] is a low-level FUSE option that is needed
as a workaround to a Linux kernel bug.
Technically this option should be set to \f[C]true\f[R] when expecting
to use mergerfs with NFS but it turns out that at least some versions of
NFS work when not enabled.
At the moment it is unclear if this has any negative side effects but
when disabled NFS appears to work alright and the kernel no longer sends
invalid requests.
When the issue is resolved in the kernel these docs will be updated with
appropriate details.
.PP
Also see Kernel Issues &
Bugs (https://github.com/trapexit/mergerfs/wiki/Kernel-Issues-&-Bugs)
for more details.
.SS SMB / CIFS
.PP
SMB (https://en.wikipedia.org/wiki/Server_Message_Block) is a protocol
most used by Microsoft Windows systems to share file shares, printers,
etc.
However, due to the popularity for Windows, it is also supported on many
other platforms including Linux.
The most popular way of supporting SMB on Linux is via the software
Samba.
.PP
Samba (https://en.wikipedia.org/wiki/Samba_(software)), and other ways
of serving Linux filesystems, via SMB should work fine with mergerfs.
The services do not tend to use the same technologies which NFS uses and
therefore don\[cq]t have the same issues.
There should not be an special settings required to use mergerfs with
Samba.
However, CIFSD (https://en.wikipedia.org/wiki/CIFSD) and other programs
have not been extensively tested.
If you use mergerfs with CIFSD or other SMB servers please submit your
experiences so these docs can be updated.
.SS SSHFS
.PP
SSHFS (https://en.wikipedia.org/wiki/SSHFS) is a FUSE filesystem
leveraging SSH as the connection and transport layer.
While often simpler to setup when compared to NFS or Samba the
performance can be lacking and the project is very much in maintenance
mode.
.PP
There are no known issues using sshfs with mergerfs.
You may want to use the following arguments to improve performance but
your millage may vary.
.IP \[bu] 2
\f[C]-o Ciphers=arcfour\f[R]
.IP \[bu] 2
\f[C]-o Compression=no\f[R]
.PP
More info can be found
here (https://ideatrash.net/2016/08/odds-and-ends-optimizing-sshfs-moving.html).
.SS Other
.PP
There are other remote filesystems but none popularly used to serve
mergerfs.
If you use something not listed above feel free to reach out and I will
add it to the list.
.SH FAQ
.SS How well does mergerfs scale? Is it \[lq]production ready?\[rq]
.PP
@ -2780,8 +2920,8 @@ executable bit set.
A common mistake by users new to Linux is to \f[C]chmod -R 644\f[R] when
they should have \f[C]chmod -R u=rwX,go=rX\f[R].
.PP
If using a network filesystem such as NFS, SMB, CIFS (Samba) be sure to
pay close attention to anything regarding permissioning and users.
If using a network filesystem such as NFS or SMB (Samba) be sure to pay
close attention to anything regarding permissioning and users.
Root squashing and user translation for instance has bitten a few
mergerfs users.
Some of these also affect the use of mergerfs from container platforms
@ -2906,33 +3046,14 @@ mergerfs uses available space for statfs calculations.
If you\[cq]ve reserved space for root then it won\[cq]t show up.
.PP
You can remove the reserve by running: \f[C]tune2fs -m 0 <device>\f[R]
.SS Can mergerfs mounts be exported over NFS?
.PP
Yes, however if you do anything which may changes files out of band
(including for example using the \f[C]newest\f[R] policy) it will result
in \[lq]stale file handle\[rq] errors unless properly setup.
.PP
Be sure to use the following options:
.IP \[bu] 2
noforget
.IP \[bu] 2
inodecalc=path-hash
.SS Can mergerfs mounts be exported over Samba / SMB?
.PP
Yes.
While some users have reported problems it appears to always be related
to how Samba is setup in relation to permissions.
.SS Can mergerfs mounts be used over SSHFS?
.PP
Yes.
.SS I notice massive slowdowns of writes when enabling cache.files.
.PP
When file caching is enabled in any form (\f[C]cache.files!=off\f[R] or
\f[C]direct_io=false\f[R]) it will issue \f[C]getxattr\f[R] requests for
\f[C]security.capability\f[R] prior to \f[I]every single write\f[R].
When file caching is enabled in any form (\f[C]cache.files!=off\f[R]) it
will issue \f[C]getxattr\f[R] requests for \f[C]security.capability\f[R]
prior to \f[I]every single write\f[R].
This will usually result in a performance degradation, especially when
using a network filesystem (such as NFS or CIFS/SMB/Samba.)
Unfortunately at this moment the kernel is not caching the response.
using a network filesystem (such as NFS or SMB.) Unfortunately at this
moment the kernel is not caching the response.
.PP
To work around this situation mergerfs offers a few solutions.
.IP "1." 3

3
src/config.cpp
View File

@ -58,6 +58,7 @@ namespace l
IFERT("branches-mount-timeout");
IFERT("cache.symlinks");
IFERT("cache.writeback");
IFERT("export-support");
IFERT("fsname");
IFERT("fuse_msg_size");
IFERT("mount");
@ -94,6 +95,7 @@ Config::Config()
category(func),
direct_io(false),
dropcacheonclose(false),
export_support(true),
flushonclose(FlushOnClose::ENUM::OPENED_FOR_WRITE),
follow_symlinks(FollowSymlinks::ENUM::NEVER),
fsname(),
@ -149,6 +151,7 @@ Config::Config()
_map["category.search"] = &category.search;
_map["direct_io"] = &direct_io;
_map["dropcacheonclose"] = &dropcacheonclose;
_map["export-support"] = &export_support;
_map["flush-on-close"] = &flushonclose;
_map["follow-symlinks"] = &follow_symlinks;
_map["fsname"] = &fsname;

1
src/config.hpp
View File

@ -118,6 +118,7 @@ public:
Categories category;
ConfigBOOL direct_io;
ConfigBOOL dropcacheonclose;
ConfigBOOL export_support;
FlushOnClose flushonclose;
FollowSymlinks follow_symlinks;
ConfigSTR fsname;

3
src/fuse_init.cpp
View File

@ -145,11 +145,12 @@ namespace FUSE
l::want_if_capable(conn_,FUSE_CAP_BIG_WRITES);
l::want_if_capable(conn_,FUSE_CAP_CACHE_SYMLINKS,&cfg->cache_symlinks);
l::want_if_capable(conn_,FUSE_CAP_DONT_MASK);
l::want_if_capable(conn_,FUSE_CAP_EXPORT_SUPPORT,&cfg->export_support);
l::want_if_capable(conn_,FUSE_CAP_IOCTL_DIR);
l::want_if_capable(conn_,FUSE_CAP_PARALLEL_DIROPS);
l::want_if_capable(conn_,FUSE_CAP_POSIX_ACL,&cfg->posix_acl);
l::want_if_capable(conn_,FUSE_CAP_READDIR_PLUS,&cfg->readdirplus);
// l::want_if_capable(conn_,FUSE_CAP_READDIR_PLUS_AUTO);
l::want_if_capable(conn_,FUSE_CAP_POSIX_ACL,&cfg->posix_acl);
l::want_if_capable(conn_,FUSE_CAP_WRITEBACK_CACHE,&cfg->writeback_cache);
l::want_if_capable_max_pages(conn_,cfg);
conn_->want &= ~FUSE_CAP_POSIX_LOCKS;