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

[Shaohua Li <shaohua.li-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Mon, 2011-01-10 at 22:26 +0800, Wu, Fengguang wrote:
> Shaohua,
> 
> On Tue, Jan 04, 2011 at 01:40:30PM +0800, Li, Shaohua wrote:
> > 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
> 
> If that is the main obstacle, why not do straightforward fincore()/
> fadvise(), and add ioctls to btrfs to export/grab the hidden
> btree_inode in any form?  This will address btrfs' specific issue, and
> have the benefit of making the VFS part general enough. You know
> ext2/3/4 already have block_dev ready for metadata readahead.
I forgot to update this comment. Please see patch 2 and patch 4, both
incore and readahead need btrfs specific staff involved, so we can't use
generic fincore or something.

Thanks,
Shaohua
> > 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!
> > 
> > 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
> > 
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