Hi, We have file readahead to do asyn file read, but has no metadata readahead. For a list of files, their metadata is stored in fragmented disk space and metadata read is a sync operation, which impacts the efficiency of readahead much. The patches try to add meatadata readahead for btrfs. It has two advantages. One is make metadata read async, the other is significant reducing disk I/O seek. In btrfs, metadata is stored in btree_inode. Ideally, if we could hook the inode to a fd so we could use existing syscalls (readahead, mincore or upcoming fincore) to do readahead, but the inode is hidden, there is no easy way for this from my understanding. Another problem is we need check page referenced bit to make sure if a page is valid, which isn't ok doing this in fincore/mincore. And in metadata readahead, filesystem need specific checking like the patch4. Doing the checking in current API (for example fadvise) will mess things too. So we add two ioctls for this. One is like readahead syscall, the other is like micore/fincore syscall. Under a harddisk based netbook with Meego, the metadata readahead reduced about 3.5s boot time in average from total 16s. v2->v3: 1. fixed some issues Arnd pointed out 2. rebased to latest git 3. remove the 'updated' page flag check from patch 2 as suggested by Fengguang. v1->v2: 1. Added more comments and fix return values suggested by Andrew Morton 2. fix a race condition pointed out by Yan Zheng initial post: http://marc.info/?l=linux-fsdevel&m=129222493406353&w=2 Thanks, Shaohua -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
