Re: [RFC][PATCH 0/5] Fiemap, an extent mapping ioctl

[Chris Mason <chris.mason@oracle.com>]

On Sunday 25 May 2008, Christoph Hellwig wrote:

Thanks for doing this Mark ;)

> On Sat, May 24, 2008 at 05:01:48PM -0700, Mark Fasheh wrote:
> > * FIEMAP_FLAG_HSM_READ
> > If the extent is offline, retrieve it before mapping and do not flag
> > it as FIEMAP_EXTENT_SECONDARY. This flag has no effect if the file
> > system does not support HSM.
>
> Given that there's no HSM support in mainline this should not be added.
> It'll be useful once we add proper HSM support, though :)
>

The HSM flag doesn't hurt, and it allows the people actually shipping hsm 
patches to use fiemap without extending the api themselves.  Reserving the 
flag isn't a bad idea.

> > * FIEMAP_FLAG_LUN_ORDER
> > If the file system stripes file data, this will return contiguous
> > regions of physical allocation, sorted by LUN. Logical offsets may not
> > make sense if this flag is passed. If the file system does not support
> > multiple LUNs, this flag will be ignored.
>
> A LUN doesn't make any sense in filesystem context.  That's a
> scsi-centric acronym that doesn't even make sense in a scsi-centric
> filesystem universe because a LUN can of course contain multiple
> partitions.  It's also extremly ill-defined when using volume managers.
>
> There's also no filesystems that actually support a single file on
> multiple device in mainline, the only filesystem that supports multiple
> data devices at all (XFS) requires each file to be on a single device.
>
> Once we have a filesystem with real multiple data device support like
> btrfs or a future XFS version we can worry about this and defined
> a different ioctl for it.
>

For btrfs I would return the logical extents via fiemap (just like the file 
were on lvm) and make btrfs specific ioctls for details about where the file 
actually lived.

fiemap alone isn't a great way to describe raid levels or complex storage 
topologies.  To include physical information I would also have to encode the 
raid level used and information about all the devices the data is replicated 
on (raid1/10)

-chris