Re: Regarding ordered-tag support.

[Tejun Heo <htejun@gmail.com>]

Hello, James.

James Bottomley wrote:
> 
> The fact that tasks can proceed in parallel, only the command components
> individual linked tasks must be serialised. so it supports the TCQ
> paradigm.
> 
> Really, also, linked tasks is exactly what a JFS wants because most of
> the updates that would be linked are something like update log entry;
> modify metadata (or data); remove log entry.
> 

I see your point, so it's not really a barrier anymore, but a sequence
of ordered writes with an intervening flush independent of other
unrelated writes.  Some random thoughts...

* All writes in the ordered sequence will have to be done sequentially,
but unrelated writes are not affected.  This can be overall gain.

* Can be implemented in block layer proper without using SCSI linked
task (probably with separate ->ordered_requests queue).  What would be
the advantage of SCSI linked task?

* As it's finer grained than wholesale barrier, FS layer will need to
pay more attention.

Thanks for the explanation.  :-)

-- 
tejun