btrfs: start deprecation of mount option inode_cache
Estimated time of removal of the functionality is 5.11, the option will be still parsed but will have no effect. Reasons for deprecation and removal: - very poor naming choice of the mount option, it's supposed to cache and reuse the inode _numbers_, but it sounds a some generic cache for inodes - the only known usecase where this option would make sense is on a 32bit architecture where inode numbers in one subvolume would be exhausted due to 32bit inode::i_ino - the cache is stored on disk, consumes space, needs to be loaded and written back - new inode number allocation is slower due to lookups into the cache (compared to a simple increment which is the default) - uses the free-space-cache code that is going to be deprecated as well in the future Known problems: - since 2011, returning EEXIST when there's not enough space in a page to store all checksums, see commit 4b9465cb ("Btrfs: add mount -o inode_cache") Remaining issues: - if the option was enabled, new inodes created, the option disabled again, the cache is still stored on the devices and there's currently no way to remove it Signed-off-by:David Sterba <dsterba@suse.com>
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