Common Linux features

The Linux operating system implements a POSIX standard interfaces and API with additional interfaces. Many of them have become common in other filesystems. The ones listed below have been added relatively recently and are considered interesting for users:

birth/origin inode time

a timestamp associated with an inode of when it was created, cannot be changed and requires the statx syscall to be read

statx

an extended version of the stat syscall that provides extensible interface to read more information that are not available in original stat

fallocate modes

the fallocate syscall allows to manipulate file extents

Supported modes:

keep size (FALLOC_FL_KEEP_SIZE)
hole punching (FALLOC_FL_PUNCH_HOLE)
zero range (FALLOC_FL_ZERO_RANGE)

FIEMAP

an ioctl that enumerates file extents, related tool is filefrag

filesystem label

another filesystem identification, could be used for mount or for better recognition, can be set or read by an ioctl or by command btrfs filesystem label

O_TMPFILE

mode of open() syscall that creates a file with no associated directory entry, which makes it impossible to be seen by other processes and is thus safe to be used as a temporary file (https://lwn.net/Articles/619146/)

xattr, acl

extended attributes (xattr) is a list of key=value pairs associated with a file, usually storing additional metadata related to security, access control list in particular (ACL) or properties (btrfs-property(8))

cross-rename

mode of renameat2 syscall that can atomically swap 2 directory entries (files/directories/subvolumes/symlinks) within the same filesystem

File attributes, XFLAGS

The btrfs filesystem supports setting file attributes or flags. Note there are old and new interfaces, with confusing names. The following list should clarify that:

attributes: chattr(1) or lsattr(1) utilities (the ioctls are FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the attributes are also called flags
xflags: to distinguish from the previous, it’s extended flags, with tunable bits similar to the attributes but extensible and new bits will be added in the future (the ioctls are FS_IOC_FSGETXATTR and FS_IOC_FSSETXATTR but they are not related to extended attributes that are also called xattrs), there’s no standard tool to change the bits, there’s support in xfs_io(8) as command xfs_io -c chattr

Attributes

a

append only, new writes are always written at the end of the file

A

no atime updates

c

compress data, all data written after this attribute is set will be compressed. Please note that compression is also affected by the mount options or the parent directory attributes.

When set on a directory, all newly created files will inherit this attribute. This attribute cannot be set with ‘m’ at the same time.

C

no copy-on-write, file data modifications are done in-place

When set on a directory, all newly created files will inherit this attribute.

Note

Due to implementation limitations, this flag can be set/unset only on empty files.

d

no dump, makes sense with 3rd party tools like dump(8), on BTRFS the attribute can be set/unset but no other special handling is done

D

synchronous directory updates, for more details search open(2) for O_SYNC and O_DSYNC

i

immutable, no file data and metadata changes allowed even to the root user as long as this attribute is set (obviously the exception is unsetting the attribute)

m

no compression, permanently turn off compression on the given file. Any compression mount options will not affect this file. (chattr(1) support added in 1.46.2)

When set on a directory, all newly created files will inherit this attribute. This attribute cannot be set with c at the same time.

S

synchronous updates, for more details search open(2) for O_SYNC and O_DSYNC

No other attributes are supported. For the complete list please refer to the chattr(1) manual page.

XFLAGS

There’s an overlap of letters assigned to the bits with the attributes, this list refers to what xfs_io(8) provides:

i: immutable, same as the attribute
a: append only, same as the attribute
s: synchronous updates, same as the attribute S
A: no atime updates, same as the attribute
d: no dump, same as the attribute