Re: [RFC] add FIEMAP ioctl to efficiently map file allocation

[David Chinner <dgc@sgi.com>]

On Tue, May 01, 2007 at 03:30:40PM -0700, Andreas Dilger wrote:
> On May 01, 2007  14:22 +1000, David Chinner wrote:
> > On Mon, Apr 30, 2007 at 04:44:01PM -0600, Andreas Dilger wrote:
> > > Hmm, I'd thought "offline" would migrate to EXTENT_UNKNOWN, but I didn't
> > 
> > I disagree - why would you want to indicate the state is unknown when we know
> > very well that it is offline?
> 
> If you don't like "UNKNOWN", what about "UNMAPPED"?  I just want a
> catch-all flag that indicates "this extent contains data but there is
> nothing sensible to be returned for the extent mapping."

Yes, I like that much more. Good suggestion. ;)

> > Effectively, when your extent is offline in the HSM, it is inaccessable, and
> > you have to bring it back from tape so it becomes accessible again. i.e. some
> > action is necessary on behalf of the user to make it accessible. So I think
> > that OFFLINE is a good name for this state because it really is inaccessible.
> 
> What you are calling OFFLINE I would prefer to call UNMAPPED, since that
> can be used by applications as a catch-all for "no mapping".  There can
> be further flags that give refinements to UNMAPPED that some applications
> might care about them (e.g. HSM_RESIDENT), but many users/apps will not
> if they just want the number of fragments in a given file.

Agreed - UNMAPPED does make a lot more sense in this case.

> > > Can you propose reasonable flag names for these (I can't think of anything
> > > very good) and a clear explanation of what they mean.  I suspect it will
> > > only be XFS that uses them initially.  In mke2fs and ext4+mballoc there is
> > > the concept of stripe unit and stripe width, but as yet they are not
> > > communicated between the two very well.  I'd be much happier if this info
> > > could be queried in a standard way from the block layer instead of the
> > > user having to specify it and the filesystem having to track it.
> > 
> > My preference is definitely for a separate ioctl to grab the
> > filesystem geometry so this stuff can be calculated in userspace.
> > i.e. the way XFS does it right now (XFS_IOC_FSGEOMETRY). I won't
> > bother trying to define names until we decide which appraoch we take
> > to implement this.
> 
> Hmm, previously you wrote "This information could be easily passed up in the
> flags fields if the filesystem has geometry information".  So, I _think_
> what you are saying is that you want 4 flags to convey this start/end
> alignment information, but the exact semantics of what a "stripe unit" and
> a "stripe width" is filesystem specific?

Right.

> I definitely do NOT want to get into any issues of querying the block
> device geometry here.  I was just making a passing comment that ext4+mballoc
> can already do RAID-specific allocation alignment, but it depends on the
> admin to specify this information and it would be nice if there was some
> easy way to get this from userspace/kernel interfaces.
> 
> Having an API that can request "tell me the number of blocks from this
> offset until the next physical disk boundary" or similar would be useful
> to any allocator, and the block layer already needs to know this when
> submitting IO.

The block layer knows this once you get inside the volume manager. I
think the issue is that there is no common export interface for this
information.

> > In XFS, mkfs.xfs does the work of getting this information
> > to see in the filesystem superblock. Here's the code for getting
> > sunit/swidth from the underlying block device:
> > 
> > http://oss.sgi.com/cgi-bin/cvsweb.cgi/xfs-cmds/xfsprogs/libdisk/
> > 
> > Not much in common there ;)
> 
> It looks like this might be just what e2fsprogs needs also.

More than likely.

> > > It does make sense to specify zero for the fm_extent_count array and a
> > > new FIEMAP_FLAG_NO_EXTENTS to return only the count of extents and not the
> > > extent data itself, for the non-verbose mode of filefrag, and for
> > > pre-allocating a buffer large enough to hold the file if that is important.
> > 
> > Rather than rely on implicit behaviour of "pass in extent count of
> > zero and a don't try to return any extents" to return the number of
> > extents on the file, why not just explicitly define this as a valid
> > input flag? i.e. FIEMAP_FLAG_GET_NUMEXTENTS
> 
> That's what I said, isn't it?  FIEMAP_FLAG_NO_EXTENTS.  I wonder if my
> clever-clever for "return no extents" and "return number of extents"
> is wasted :-/.

Too clever for an API, I think. ;)

My point is mainly that if you are going to use an API for a
specific function (e.g. query the number of extents) I think that
the API should have an obvious method for executing that specific
function. Using a command of "get no extents" to provide the query
of "how many extents in this file" is kind of obscure. When you read
the code it doesn't make a lot of sense, as opposed to seeing a
clear statement of intent from the code itself.

i.e. FIEMAP_FLAG_GET_NUMEXTENTS is self-documenting in both the API
and the code that uses it...

> > > - does XFS return an extent for the metadata parts of the file (e.g. btree)?
> > 
> > No, but we can return the extent map for the attribute fork (i.e.
> > extended attrs) if asked for (XFS_IOC_GETBMAPA).
> 
> This seems like it would be a useful addition to the interface also, having
> FIEMAP_FLAG_METADATA request the return of metadata allocations too.

Agreed. The different types of requests need to be mutually
exclusive, though - returning the map of the attribute fork mixed
with the map of the data fork is going to be confusing....

> > > - does XFS allow non-root users to call xfs_bmap on files they don't own, or
> > >   use by non-root users at all?
> > 
> > Users can run xfs_bmap on any file they have permission to
> > open(O_RDONLY).
> > 
> > >   The FIBMAP ioctl is for privileged users
> > >   only, and I wonder if FIEMAP should be the same, or at least disallow
> > >   mapping files that the user can't access especially with FLAG_SYNC and/or
> > >   FLAG_HSM_READ.
> > 
> > I see little reason for restricting FI[BE]MAP to privileged users -
> > anyone should be able to determine if files they have permission to
> > access are fragmented.
> 
> I think I agree with Anton that allowing some of the flags for non-privileged
> users seems dangerous.  I think this needs to be determined on a flag-by-flag
> basis, and -EPERM should be returned in some cases.

Agreed, but I'm yet to see any flags where I think that is necessary
yet.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group