From: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxxxx> Introduces the encoding-awareness and 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 | 41 ++++++++++++++++++++++++++++++ 1 file changed, 41 insertions(+) diff --git a/Documentation/admin-guide/ext4.rst b/Documentation/admin-guide/ext4.rst index e506d3dae510..4e08d0309f1e 100644 --- a/Documentation/admin-guide/ext4.rst +++ b/Documentation/admin-guide/ext4.rst @@ -91,10 +91,51 @@ Currently Available * large block (up to pagesize) support * efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force the ordering) +* Encoding aware file names +* 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. +Encoding-aware file names and case-insensitive lookups +====================================================== + +Ext4 optionally supports filesystem-wide charset knowledge when handling +file names, which allows the user to perform file system lookups using +charset equivalent versions of the same file name, and optionally ensure +that no invalid names are held by the filesystem. charset encoding +awareness is also essential for performing case-insensitive lookups, +because it is what defines the casefold operation. + +The case-insensitive file name lookup feature is supported in a smaller +granularity, 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 a file attribute on an empty directory. For +the reason stated above, the filesystem must have encoding enabled to +use this feature. + +Both encoding-awareness and case-awareness are 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 directories with +DX feature. On DX directories, the hash must be calculated using the +normalized 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 and equivalent sequence +lookups won't work. + Options ======= -- 2.20.1
