Index

• About
• Usage
• Installation
• Use Cases
• Keeping data
• Mount options

nullfs

A virtual file system that behaves like /dev/null

It can handle regular file operations like mkdir/rmdir/ln but writing to files does not store any data. The file size is however saved, reading from the files behaves like reading from /dev/zero.

Writing and reading is basically an NOOP, so it can be used for performance testing with applications that require directory structures. Implemented as kernel module, instead of using FUSE, there is absolutely no overhead for copying application data from user to kernel space while performing write or read operations.

usage

# make
make -C /lib/modules/4.18.5/build M=/home/abi/lwnfs modules
make[1]: Entering directory '/usr/src/linux-headers-4.18.5'
  Building modules, stage 2.
  MODPOST 1 modules
make[1]: Leaving directory '/usr/src/linux-headers-4.18.5'

# insmod nullfs.ko 
# mkdir /sinkhole
# mount -t nullfs none /sinkhole/
# mkdir /sinkhole/testdir
# touch /sinkhole/testdir/myfile
# echo foobar > /sinkhole/testdir/myfile
# ls -lah /sinkhole/testdir/myfile
-rw-r--r-- 1 root root 0 Nov  8 20:17 /sinkhole/testdir/myfile
# cat /sinkhole/testdir/myfile
# pv < /dev/zero > /sinkhole/testdir/myfile
11.1GiB 0:00:04 [3.85GiB/s] [     <=>      ] 
# cat /sinkhole/testdir/myfile
# 

File size is preserved to work around applications that do size checks:

# dd if=/dev/zero of=/nullfs/DATA bs=1M count=20
# 20+0 records in
# 20+0 records out
# 20971520 bytes (21 MB, 20 MiB) copied, 0.00392452 s, 5.3 GB/s
# stat -c%s /nullfs/DATA
# 20971520

Reading from the files does not copy anything to userspace and is an NOOP; makes it behave like reading from /dev/zero:

# dd if=/nullfs/DATA of=/tmp/REALFILE
# 40960+0 records in
# 40960+0 records out
# 20971520 bytes (21 MB, 20 MiB) copied, 0.0455288 s, 461 MB/s
# hexdump -C /tmp/REALFILE
# 00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|

installation

To install the module for the running linux kernel use:

 # make -C /lib/modules/$(uname -r)/build M=$PWD modules_install INSTALL_MOD_DIR=kernel/fs/nullfs
 # depmod

Running depmod is mandatory. Now the module can be loaded via:

 modprobe  nullfs

To automatically load filesystem module during boot time, create a configuration file suitable for your distribution, usually located in /etc/modules-load.d

echo nullfs > /etc/modules-load.d/nullfs.conf

Example entry for /etc/fstab, mounting the filesystem to /nullfs:

none    /nullfs nullfs auto

keep

There is the possiblity to exclude certain files from beeing sent into the void. For example if the file matching "fstab" should be kept in memory, one can mount nullfs with the "write=" option.

# mount -t nullfs none /sinkhole/ -o write=fstab
# cp /etc/fstab /sinkhole/
# wc -l /sinkhole/fstab 
14 /sinkhole/fstab
# cp /etc/passwd /sinkhole/
# wc -l /sinkhole/passwd 
0 /sinkhole/passwd

Another option is using the sysfs interface to change the exclude string after the module has been loaded:

 echo foo  > /sys/fs/nullfs/exclude 

Keep in mind that file data is kept in memory and no boundary checks are done, so this might fill up your RAM in case you exclude big files from beeing nulled.

ACL

It is possible to set POSIX ACL attributes via setfacl so it appears the filesystem supports them, they are not saved.

Works with recent linux kernels (5.x), nullfs builds fine with older kernels (4.x, 3.x) but setting ACL information fails with "Operation not supported".

usecases

See: Use Cases

The module has been used for performance testing with redis, see:

https://www.usenix.org/system/files/atc20-george.pdf

Please report usecases as regular issues!

supported

The following mount options are supported:

 -o mode=      set permissions on mount directory ( mount .. -o mode=777 )
 -o uid=       set uid on mount directory ( mount .. -o uid=1000 )
 -o gid=       set gid on mount directory ( mount .. -o gid=1000 )
 

todos/ideas

• replace simple_statfs call with real one, show free space of a directory that can be passed during kernel module load
• support multiple parameters for write= option
• allow regex for write= option via trace.h
• simulate xattr support?