Re: Topology ioctls

[Jamie Lokier <jamie@shareable.org>]

Martin K. Petersen wrote:
> 
> The original rationale for exporting the topology information via sysfs
> was that we intended to support multiple heterogeneous regions within a
> block device.  And that fit poorly with an ioctl approach.
> 
> However, with a single region per device it is trivial to provide the
> topology.  And while mkfs.* will continue to use the libblkid interface,
> there are users that would like to get access to this information
> without having to traverse sysfs and stitch things together manually.

Quite nice, I can see it coming in handy.

One more bit of information I'd like is the "write affected block
size".  When you write to a single disk, it's the sector size (512 or
soon to be 4096).  For a RAID, it's probably quite large, depending on
implementation details.  For some kind of flash - see threads from
Pavel and LWN about sector writes causing erase-block sizes to be
lost.

That size is useful to any program which does journalling/logging type
of write pattern, i.e. databases and filesystems-in-a-file, so they
can write new commit blocks sufficiently far apart.

-- Jamie