Re: [PATCH v2 0/5] add new ioctls to do metadata readahead in btrfs

On Wed, 2011-01-05 at 00:14 +0800, Jeff Moyer wrote:

Shaohua Li [off-list ref] writes:

quoted

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.
  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. 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.
  Last time I posted similar patches to btrfs maillist, which adds the
new ioctls in btrfs specific ioctl code. But Christoph Hellwig asks we
have a generic interface to do this so other filesystem can share some
code, so I came up with the new one. Comments and suggestions are
welcome!

Is it not possible to enhance the existing readahead mechanisms to work
on metadata as well? 

using existing sys_readahead to do metadata readahead? The problem is I
can't hook a fd for meatadata inode, as explained above.
or let kernel automatically do metadata readahead? Kernel can't be so
intelligent, because kernel doesn't even know which file should be
readahead till userspace tells it

 Is there some reason why metadata should be
fetched separately from the data it references?

metadata read is sync operation, which will break data readahead
pipeline. And metadata and data usually lives in not adjacent disk
blocks, which will introduce a lot of disk seeks. reading metadata first
and then do data readahead can reduce a lot of disk seeks and data
readahead can be fully pumped

Thanks,
Shaohua