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Tahsin Erdogan
dec214d00e
Ext4 now supports xattr values that are up to 64k in size (vfs limit). Large xattr values are stored in external inodes each one holding a single value. Once written the data blocks of these inodes are immutable. The real world use cases are expected to have a lot of value duplication such as inherited acls etc. To reduce data duplication on disk, this patch implements a deduplicator that allows sharing of xattr inodes. The deduplication is based on an in-memory hash lookup that is a best effort sharing scheme. When a xattr inode is read from disk (i.e. getxattr() call), its crc32c hash is added to a hash table. Before creating a new xattr inode for a value being set, the hash table is checked to see if an existing inode holds an identical value. If such an inode is found, the ref count on that inode is incremented. On value removal the ref count is decremented and if it reaches zero the inode is deleted. The quota charging for such inodes is manually managed. Every reference holder is charged the full size as if there was no sharing happening. This is consistent with how xattr blocks are also charged. [ Fixed up journal credits calculation to handle inline data and the rare case where an shared xattr block can get freed when two thread race on breaking the xattr block sharing. --tytso ] Signed-off-by: Tahsin Erdogan <[email protected]> Signed-off-by: Theodore Ts'o <[email protected]>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.