On 04/20/2012 05:45 AM, Ted Ts'o wrote:

The other approach is to leave things roughly undefined, and accept
the fact that applications which use this will probably be specialized
applications that are very much aware of what file system they are
using, 

If that is the case then I prefer an FS specific IOCTL. Since the app
already has FS specific code built in.

and just need to pass minimal hints to the application in a
general way, and that's the approach I went with in this O_HOT/O_COLD proposal.

You are contradicting yourself. Above you say specific FS (read ext4)
and here you say "general way".

Please show me how your proposal is not ext4 outer-rim specific, in devices
that are single rotational disks.

What does the "general way" mean?

I suspect that HOT/COLD is enough to go quite far even for tiered
storage; maybe at some point we will want some other, more
fine-grained interface where an application program can very precisely
dial in their requirements in a T10-like fashion.  Perhaps.  But I
don't think having a simple O_HOT/O_COLD interface precludes the
other, or vice versa.  In fact, one advantage with sticking with
HOT/COLD is that there's much less chance of bike-shedding, with
people arguing over what a more fine-grained interface might look like.

But bike-shedding is exactly what you propose. (well not that you actually
stated what you propose)

Your patch says "beginning of the disk" but the flag is called O_HOT,
That's bike-shedding. You hope there will be new meaning for it in the
future.

So why not start with this, and if we need to use something more
complex later, we can cross that bridge if and when we get to it?  In
the meantime, I think there are valid uses of this simple, minimal
interface in the case of a local disk file system supporting a cluster
file system such as Hadoopfs or TFS.  One of the useful things that
came out of the ext4 workshop where we got to talk to developers from
Taobao was finding out how much their interests matched with some of
the things we've talked about doing at Google to support our internal
customers.

This all reads, ext4 specific / app specific. Why a general API? and
why must it be at create?

	       	      	    	 	   - Ted

Thanks
Boaz