Re: Is it possible for the ext4/btrfs file system to pass some context related info to low level block driver?

[Matthew Wilcox <matthew@wil.cx>]

On Wed, May 04, 2011 at 08:51:39AM -0600, Andreas Dilger wrote:
> I was aware of REQ_META, but I didn't know there was any benefit to
> using it. I think it would be easy to set REQ_META on all ext4 metadata
> if there was a reason to do so.

The CFQ ioscheduler pays attention to it (prioritising metadata accesses
over data accesses), and blocktrace will print an 'M' for metadata
requests if it's set, so I think that's two excellent reasons to set
REQ_META today.

However, ext3, ext4, and XFS already use it:

fs/ext3/inode.c:1105:   ll_rw_block(READ_META, 1, &bh);
fs/ext3/inode.c:2754:           submit_bh(READ_META, bh);
fs/ext3/namei.c:924:                                    ll_rw_block(READ_META, 1, &bh);
fs/ext4/inode.c:1500:   ll_rw_block(READ_META, 1, &bh);
fs/ext4/inode.c:4775:           submit_bh(READ_META, bh);
fs/ext4/namei.c:924:                                    ll_rw_block(READ_META, 1, &bh);
fs/gfs2/log.c:597:              submit_bh(WRITE_SYNC | REQ_META, bh);
fs/gfs2/log.c:599:              submit_bh(WRITE_FLUSH_FUA | REQ_META, bh);
fs/gfs2/meta_io.c:39:   int write_op = REQ_META |
fs/gfs2/meta_io.c:228:  submit_bh(READ_SYNC | REQ_META, bh);
fs/gfs2/meta_io.c:435:          ll_rw_block(READ_SYNC | REQ_META, 1, &first_bh);
fs/gfs2/ops_fstype.c:221:       submit_bio(READ_SYNC | REQ_META, bio);
fs/gfs2/quota.c:710:            ll_rw_block(READ_META, 1, &bh);
fs/xfs/linux-2.6/xfs_buf.c:1321:                rw = (bp->b_flags & XBF_WRITE) ? WRITE_META : READ_META;
include/linux/fs.h:164:#define READ_META                (READ | REQ_META)
include/linux/fs.h:168:#define WRITE_META               (WRITE | REQ_META)

btrfs seems to not use REQ_META yet.  *poke* *poke* :-)

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."