On Thu, 2011-01-20 at 04:34 +0800, Andrew Morton wrote: > On Wed, 19 Jan 2011 09:15:15 +0800 > Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > > > 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. > > Has anyone looked at implementing this for filesystems other than > btrfs? Have the ext4 guys taken a look? Did they see any impediments > to implementing it for ext4? Not yet. I do expect ext4 guys can check it. From my understanding, it should be relatively easy to do it in ext filesystems. > > Under a harddisk based netbook with Meego, the metadata readahead > > reduced about 3.5s boot time in average from total 16s. > > That's a respectable speedup. And it *needs* to be a good speedup, > given how hacky all of this is! > > But then.. reducing bootup time on a laptop/desktop/server by 3.5s > isn't exactly a world-shattering benefit, is it? Is it worth all the > hacky code? a laptop/desktop/server need read more data from hard disks, this will give more bootup time saving I think, though not tested yet. > It would be much more valuable if those 3.5 seconds were available to > devices which really really care about bootup times, but very few of > those devices use rotating disks nowadays, I expect? Currently most popular netbooks are using rotating disks actually. And this will benefit laptop/desktop too. 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
