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docs: ext4.rst: document case-insensitive directories 

Introduces the case-insensitive features on ext4 for system
administrators.  Explain the minimum of design decisions that are
important for sysadmins wanting to enable this feature.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
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Gabriel Krisman Bertazi 2019-04-25 14:13:27 -04:00 committed by Theodore Ts'o
parent b886ee3e77
commit 0a790fe438
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Documentation/admin-guide/ext4.rst
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@ -91,10 +91,48 @@ 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
[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
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