From: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> --- Documentation/admin-guide/ext4.rst | 38 ++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/Documentation/admin-guide/ext4.rst b/Documentation/admin-guide/ext4.rst index e506d3dae510..9e8b35ed7cd9 100644 --- a/Documentation/admin-guide/ext4.rst +++ b/Documentation/admin-guide/ext4.rst @@ -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 +fname_encoding feature, which stores the filesystem-wide encoding +model used. By default, the charset adopted is the latest version of +Unicode (12.0.0, 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 ======= -- 2.20.1
