|Subject:||Re: [PATCH]ext4: online defrag: Enable to reuse blocks by multiple defrag|
|From:||Theodore Tso (tyt...@mit.edu)|
|Date:||Dec 8, 2008 9:46:58 pm|
On Tue, Dec 09, 2008 at 11:26:37AM +0900, Akira Fujita wrote:
I'm redesigning ext4 online defrag based on the comments from Ted. Probably defrag's block allocation method will be changed greatly.
FYI, there was a discussion about defrag on today's ext4 call. One of the ideas that was kicked around was to completely change the primitives used by defrag, and to design things around three primitive, general purpose interfaces.
We didn't go into complete detail on the call, but let me give you a strawman proposal for consideration/discussion:
(1) An (ioctl-based) interface which allows a privileged program to specify one or more range of blocks which the filesystem's block allocator must NOT allocate from. (We may want to have a flag for each block range which either makes the block lockout advisory, such that if the block allocator can't find blocks anywhere else, it may invade the reserved block area --- or mandatory, where if there are no other blocks, the filesystem returns ENOSPC). This allows the defragmenter to work on an area of the disk without worrying about concurrent allocations by other processes from getting in the way.
(2) An (ioctl-based) interface which associates with an inode preferred range(s) of blocks which the block allocator will try using first; if those blocks are not available, or the block range(s) is exhausted, the block allocator use its normal algorithms to pick the best available block. The set of preferred blocks is only guaranteed to persist while the inode is in memory.
(3) An (ioctl-based) interface which takes two inode numbers, and allows a privileged program to "defrag" an inode by using blocks from a donor inode and using them as the new blocks for the destination inode, preserving the contents of the destination inode.
The advantage of this implementation strategy is that each of the interfaces can be implemented one at a time, with very well defined semantics, and which can be independently tested. The semantics can also be used in different combinations to solve alternate problems. For example, a combination of (1) and (2) can be used to reserve blocks for use by a directory that is expected to grow, so the directory can use contiguous blocks. Or, they could be used to implement an "online shrink" that would allow a filesystem to be resized to a smaller size.
One other thing that comes to mind. If it turns out that these interfaces have multiple users, and in some cases the reservations or block allocation restrictions are expected to last for longer than a process lifetime, it may be useful to tag them with a short (8-16 character) name, so that it is possible to list the current set of reservations, and so they can be removed by a privileged user. This could be overdesigning the interface; but the whole *point* of thinking about the interfaces from a more generic point of view (as opposed for use by a specific program for which the kernel interfaces are custom-designed) is that hopefully they will have multiple use cases and multiple users, in which case we need to worry about how multiple users can co-exist.