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Diffstat (limited to 'Documentation/admin-guide/ext4.rst')
| -rw-r--r-- | Documentation/admin-guide/ext4.rst | 111 |
1 files changed, 75 insertions, 36 deletions
diff --git a/Documentation/admin-guide/ext4.rst b/Documentation/admin-guide/ext4.rst index e506d3dae510..ac0c709ea9e7 100644 --- a/Documentation/admin-guide/ext4.rst +++ b/Documentation/admin-guide/ext4.rst @@ -91,10 +91,50 @@ Currently Available * large block (up to pagesize) support * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force the ordering) +* Case-insensitive file name lookups +* file-based encryption support (fscrypt) +* file-based verity support (fsverity) [1] Filesystems with a block size of 1k may see a limit imposed by the directory hash tree having a maximum depth of two. +case-insensitive file name lookups +====================================================== + +The case-insensitive file name lookup feature is supported on a +per-directory basis, allowing the user to mix case-insensitive and +case-sensitive directories in the same filesystem. It is enabled by +flipping the +F inode attribute of an empty directory. The +case-insensitive string match operation is only defined when we know how +text in encoded in a byte sequence. For that reason, in order to enable +case-insensitive directories, the filesystem must have the +casefold feature, which stores the filesystem-wide encoding +model used. By default, the charset adopted is the latest version of +Unicode (12.1.0, by the time of this writing), encoded in the UTF-8 +form. The comparison algorithm is implemented by normalizing the +strings to the Canonical decomposition form, as defined by Unicode, +followed by a byte per byte comparison. + +The case-awareness is name-preserving on the disk, meaning that the file +name provided by userspace is a byte-per-byte match to what is actually +written in the disk. The Unicode normalization format used by the +kernel is thus an internal representation, and not exposed to the +userspace nor to the disk, with the important exception of disk hashes, +used on large case-insensitive directories with DX feature. On DX +directories, the hash must be calculated using the casefolded version of +the filename, meaning that the normalization format used actually has an +impact on where the directory entry is stored. + +When we change from viewing filenames as opaque byte sequences to seeing +them as encoded strings we need to address what happens when a program +tries to create a file with an invalid name. The Unicode subsystem +within the kernel leaves the decision of what to do in this case to the +filesystem, which select its preferred behavior by enabling/disabling +the strict mode. When Ext4 encounters one of those strings and the +filesystem did not require strict mode, it falls back to considering the +entire string as an opaque byte sequence, which still allows the user to +operate on that file, but the case-insensitive lookups won't work. + Options ======= @@ -143,14 +183,17 @@ When mounting an ext4 filesystem, the following option are accepted: system after its metadata has been committed to the journal. commit=nrsec (*) - Ext4 can be told to sync all its data and metadata every 'nrsec' - seconds. The default value is 5 seconds. This means that if you lose - your power, you will lose as much as the latest 5 seconds of work (your - filesystem will not be damaged though, thanks to the journaling). This - default value (or any low value) will hurt performance, but it's good - for data-safety. Setting it to 0 will have the same effect as leaving - it at the default (5 seconds). Setting it to very large values will - improve performance. + This setting limits the maximum age of the running transaction to + 'nrsec' seconds. The default value is 5 seconds. This means that if + you lose your power, you will lose as much as the latest 5 seconds of + metadata changes (your filesystem will not be damaged though, thanks + to the journaling). This default value (or any low value) will hurt + performance, but it's good for data-safety. Setting it to 0 will have + the same effect as leaving it at the default (5 seconds). Setting it + to very large values will improve performance. Note that due to + delayed allocation even older data can be lost on power failure since + writeback of those data begins only after time set in + /proc/sys/vm/dirty_expire_centisecs. barrier=<0|1(*)>, barrier(*), nobarrier This enables/disables the use of write barriers in the jbd code. @@ -169,16 +212,6 @@ When mounting an ext4 filesystem, the following option are accepted: that ext4's inode table readahead algorithm will pre-read into the buffer cache. The default value is 32 blocks. - nouser_xattr - Disables Extended User Attributes. See the attr(5) manual page for - more information about extended attributes. - - noacl - This option disables POSIX Access Control List support. If ACL support - is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL - is enabled by default on mount. See the acl(5) manual page for more - information about acl. - bsddf (*) Make 'df' act like BSD. @@ -205,11 +238,10 @@ When mounting an ext4 filesystem, the following option are accepted: configured using tune2fs) data_err=ignore(*) - Just print an error message if an error occurs in a file data buffer in - ordered mode. + Just print an error message if an error occurs in a file data buffer. + data_err=abort - Abort the journal if an error occurs in a file data buffer in ordered - mode. + Abort the journal if an error occurs in a file data buffer. grpid | bsdgroups New objects have the group ID of their parent. @@ -349,9 +381,16 @@ When mounting an ext4 filesystem, the following option are accepted: dax Use direct access (no page cache). See - Documentation/filesystems/dax.txt. Note that this option is + Documentation/filesystems/dax.rst. Note that this option is incompatible with data=journal. + inlinecrypt + When possible, encrypt/decrypt the contents of encrypted files using the + blk-crypto framework rather than filesystem-layer encryption. This + allows the use of inline encryption hardware. The on-disk format is + unaffected. For more details, see + Documentation/block/inline-encryption.rst. + Data Mode ========= There are 3 different data modes: @@ -359,7 +398,7 @@ There are 3 different data modes: * writeback mode In data=writeback mode, ext4 does not journal data at all. This mode provides - a similar level of journaling as that of XFS, JFS, and ReiserFS in its default + a similar level of journaling as that of XFS and JFS in its default mode - metadata journaling. A crash+recovery can cause incorrect data to appear in files which were written shortly before the crash. This mode will typically provide the best ext4 performance. @@ -479,21 +518,21 @@ Files in /sys/fs/ext4/<devname>: Ioctls ====== -There is some Ext4 specific functionality which can be accessed by applications -through the system call interfaces. The list of all Ext4 specific ioctls are -shown in the table below. +Ext4 implements various ioctls which can be used by applications to access +ext4-specific functionality. An incomplete list of these ioctls is shown in the +table below. This list includes truly ext4-specific ioctls (``EXT4_IOC_*``) as +well as ioctls that may have been ext4-specific originally but are now supported +by some other filesystem(s) too (``FS_IOC_*``). -Table of Ext4 specific ioctls +Table of Ext4 ioctls - EXT4_IOC_GETFLAGS + FS_IOC_GETFLAGS Get additional attributes associated with inode. The ioctl argument is - an integer bitfield, with bit values described in ext4.h. This ioctl is - an alias for FS_IOC_GETFLAGS. + an integer bitfield, with bit values described in ext4.h. - EXT4_IOC_SETFLAGS + FS_IOC_SETFLAGS Set additional attributes associated with inode. The ioctl argument is - an integer bitfield, with bit values described in ext4.h. This ioctl is - an alias for FS_IOC_SETFLAGS. + an integer bitfield, with bit values described in ext4.h. EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD Get the inode i_generation number stored for each inode. The @@ -568,7 +607,7 @@ kernel source: <file:fs/ext4/> programs: http://e2fsprogs.sourceforge.net/ -useful links: http://fedoraproject.org/wiki/ext3-devel +useful links: https://fedoraproject.org/wiki/ext3-devel http://www.bullopensource.org/ext4/ http://ext4.wiki.kernel.org/index.php/Main_Page - http://fedoraproject.org/wiki/Features/Ext4 + https://fedoraproject.org/wiki/Features/Ext4 |