Hi Ivan, I think that everything should be quite simple. Let the FITRIM ioctl spawn a process, which scans all the bitmap blocks and puts the free-space-extents to the deferred delete_set. At the same time, we mark them as "busy" in the WORKING bitmap to make sure that nobody will touch them (the post_commit_hook will make them clean again). Basically, that's all: our transaction manager and the discard code will do all the other work :) Details First, the trim process reads up a bitmap node and puts it to the transaction (see try_capture() and friends). Then we go from left to right in the region of blocks covered by the bitmap node and handle the "free extents". In every such iteration we lock the bitmap node, locate the free extent, mark it dirty in the WORKING bitmap, put the respective entry to the deferred delete_set and unlock the bitmap node. NOTE that commit of atoms spawned by the trim process will be unusual: no dirty jnodes, hence, no flush, no IOs (as RELOCATE and OVERWRITE sets are empty). Only issuing discard requests.. Of course, this is only in the case when other processes spawning dirty jnodes didn't join our atom (they can do it perfectly, as we lock bitmaps only per a free extent handling). There can be potential problems around the "unusual" commits (false positives in the debugging code, etc.), but I hope they are minor.. Thanks, Edward. On 06/30/2014 11:35 AM, Ivan Shapovalov wrote:
Hi, I've started to think about implementing the second part of discard support, namely "batch mode" (FITRIM ioctl). And it seems like I don't yet quite understand how to do it. It had been suggested to reuse existing transaction and discard machinery for this feature: create a transaction, allocate+deallocate all possible blocks and then force-commit it. However, the algorithms of discard_atom() are very inoptimal for discarding large amounts of known free space -- a bitmap check is performed for every single discard unit. Repeatedly calling reiser4_alloc_blocks() to allocate every possible block also seems inefficient. And this will still miss those 10% of reserved space, IIUC. So the best way I can imagine is to introduce a new space allocator method, "iterate free space", and discard all reported extents (blkdev_issue_discard() will take care of aligning them properly). With such method, a question arises: how to prevent bitmap modifications and disk writes to free space when such iteration is in progress? Thanks,
-- To unsubscribe from this list: send the line "unsubscribe reiserfs-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
