Re: [PATCH 0/4] Fiemap, an extent mapping ioctl - round 2

[Andreas Dilger <adilger@sun.com>]

On Jun 26, 2008  10:36 +0100, Jamie Lokier wrote:
> Mark Fasheh wrote:
> > * FIEMAP_FLAG_SYNC
> > If this flag is set, the kernel will sync the file before mapping extents.
> 
> Is there a reason why fsync() before calling FIEMAP is unsuitable?

This was added because the xfsbmap operation always did an fsync before
returning the extents.  I don't think it is strictly required, but it
isn't harmful either.

> > * FIEMAP_FLAG_XATTR
> > If this flag is set, the extents returned will describe the inodes
> > extended attribute lookup tree, instead of it's data tree.
> 
> What is this for?  The meaning of the xattr tree sounds rather
> filesystem specific to me.

This is to return the location of the xattr blocks for the inode.

> > * FIEMAP_EXTENT_NO_DIRECT
> > Direct access to the data in this extent is illegal or will have
> > undefined results.
> ...
> > * FIEMAP_EXTENT_NET
> >   - This will also set FIEMAP_EXTENT_NO_DIRECT
> > The data for this extent is not stored in a locally-accessible device.
> 
> Does this _always_ set FIEMAP_EXTENT_NO_DIRECT?  Some network
> filesystems do support O_DIRECT access - NFS comes to mind.
> 
> (I'm assuming 'direct access' means O_DIRECT).

"NO_DIRECT" has nothing to do with "O_DIRECT".  It just means that,
per the description a few lines earlier, direct access to the file
data is impossible (i.e. for lilo or other tool which thinks it can
open "dev" and seek to "fe_physical" to read the data), or at best
will have undefined results (e.g. you may get encrypted or compressed
data back, or it is on the far side of a network interface).

> > * FIEMAP_EXTENT_DATA_ENCRYPTED
> >   - This will also set FIEMAP_EXTENT_NO_DIRECT
> > The data in this extent has been encrypted by the file system.
> 
> I don't think encryption necessarily rules out O_DIRECT.  It'll depend
> how I/O is implemented by that filesystem.
> 
> > * FIEMAP_EXTENT_DATA_INLINE
> >   This will also set FIEMAP_EXTENT_NOT_ALIGNED
> > Data is located within a meta data block.
> 
> This seems like it would always set FIEMAP_EXTENT_NO_DIRECT :-)
> 
> (Generally, won't FIEMAP_EXTENT_NOT_ALIGNED always set
> FIEMAP_EXTENT_NO_DIRECT?)
> 
> > * FIEMAP_EXTENT_DATA_TAIL
> >   This will also set FIEMAP_EXTENT_NOT_ALIGNED
> > Data is packed into a block with data from other files.
> 
> Maybe this too.

The rest of these comments seem based on the previous misunderstanding.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.