This is a giant comment at the top of block-rsv.c describing generally how block rsvs work. It is purely about the block rsv's themselves, and nothing to do with how the actual reservation system works. Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx> --- fs/btrfs/block-rsv.c | 81 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 81 insertions(+) diff --git a/fs/btrfs/block-rsv.c b/fs/btrfs/block-rsv.c index d07bd41a7c1e..54380f477f80 100644 --- a/fs/btrfs/block-rsv.c +++ b/fs/btrfs/block-rsv.c @@ -6,6 +6,87 @@ #include "space-info.h" #include "transaction.h" +/* + * HOW DO BLOCK RSVS WORK + * + * Think of block_rsv's as bucktes for logically grouped reservations. Each + * block_rsv has a ->size and a ->reserved. ->size is how large we want our + * block rsv to be, ->reserved is how much space is currently reserved for + * this block reserve. + * + * ->failfast exists for the truncate case, and is described below. + * + * NORMAL OPERATION + * We determine we need N items of reservation, we use the appropriate + * btrfs_calc*() helper to determine the number of bytes. We call into + * reserve_metadata_bytes() and get our bytes, we then add this space to our + * ->size and our ->reserved. + * + * We go to modify the tree for our operation, we allocate a tree block, which + * calls btrfs_use_block_rsv(), and subtracts nodesize from + * block_rsv->reserved. + * + * We finish our operation, we subtract our original reservation from ->size, + * and then we subtract ->size from ->reserved if there is an excess and free + * the excess back to the space info, by reducing space_info->bytes_may_use by + * the excess amount. + * + * In some cases we may return this excess to the global block reserve or + * delayed refs reserve if either of their ->size is greater than their + * ->reserved. + * + * BLOCK_RSV_TRANS, BLOCK_RSV_DELOPS, BLOCK_RSV_CHUNK + * These behave normally, as described above, just within the confines of the + * lifetime of ther particular operation (transaction for the whole trans + * handle lifetime, for example). + * + * BLOCK_RSV_GLOBAL + * This has existed forever, with diminishing degrees of importance. + * Currently it exists to save us from ourselves. We definitely over-reserve + * space most of the time, but the nature of COW is that we do not know how + * much space we may need to use for any given operation. This is + * particularly true about the extent tree. Modifying one extent could + * balloon into 1000 modifications of the extent tree, which we have no way of + * properly predicting. To cover this case we have the global reserve act as + * the "root" space to allow us to not abort the transaciton when things are + * very tight. As such we tend to treat this space as sacred, and only use it + * if we are desparate. Generally we should no longer be depending on its + * space, and if new use cases arise we need to address them elsewhere. + * + * BLOCK_RSV_DELALLOC + * The individual item sizes are determined by the per-inode size + * calculations, which are described with the delalloc code. This is pretty + * straightforward, it's just the calculation of ->size encodes a lot of + * different items, and thus it gets used when updating inodes, inserting file + * extents, and inserting checksums. + * + * BLOCK_RSV_DELREFS + * We keep a running talley of how many delayed refs we have on the system. + * We assume each one of these delayed refs are going to use a full + * reservation. We use the transaction items and pre-reserve space for every + * operation, and use this reservation to refill any gap between ->size and + * ->reserved that may exist. + * + * From there it's straightforward, removing a delayed ref means we remove its + * count from ->size and free up reservations as necessary. Since this is the + * most dynamic block rsv in the system, we will try to refill this block rsv + * first with any excess returned by any other block reserve. + * + * BLOCK_RSV_EMPTY + * This is the fallback block rsv to make us try to reserve space if we don't + * have a specific bucket for this allocation. It is mostly used for updating + * the device tree and such, since that is a separate pool we're content to + * just reserve space from the space_info on demand. + * + * BLOCK_RSV_TEMP + * This is used by things like truncate and iput. We will temporarily + * allocate a block rsv, set it to some size, and then truncate bytes until we + * have no space left. With ->failfast set we'll simply return ENOSPC from + * btrfs_use_block_rsv() to signal that we need to unwind and try to make a + * new reservation. This is because these operations are unbounded, so we + * want to do as much work as we can, and then back off and re-reserve. + */ + static u64 block_rsv_release_bytes(struct btrfs_fs_info *fs_info, struct btrfs_block_rsv *block_rsv, struct btrfs_block_rsv *dest, u64 num_bytes, -- 2.24.1
