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Thu Sep 18 10:49:04 PDT 2008

should be passed to the I/O schedulers, since the maximum number
of request descriptors is limited. 

And I still don't understand if you want to make your rb-tree
work efficiently, you need to put a lot of bios or request descriptors
into the tree. Is that what you are going to do?
On the other hand, dm-ioband tries to minimize to have bios blocked.
And I have a plan on reducing the maximum number that can be
blocked there.

Sorry to bother you that I just don't understand the concept clearly.

> So basically mechanism of maintaining an rb-tree can be completely
> ignorant of the fact whether a driver is keeping track of bios or keeping
> track of requests per cgroup. 

I don't care whether the queue is implemented as a rb-tee or some
kind of list because they are logically the same thing.

> > When one of the dm drivers accepts a I/O request, the request
> > won't have either a real device number or a real sector number.
> > The request will be re-mapped to another sector of another device
> > in every dm drivers. The request may even be replicated there.
> > So it is really hard to find the right request queue to put
> > the request into and sort them on the queue.
> Hmm.., I thought that all the incoming requests to dm/md driver will
> remain in a single queue maintained by that drvier (irrespective of the
> fact in which request queue these requests go in lower layers after
> replication or other operation). I am not very familiar with dm/md
> implementation. I will read more about it....

They never look into the queues maintained in drivers.
Some of them have its own little queue and others don't.
Some may just modify the sector numbers of I/O requests or may
create a new I/O request themselves. Others such as md-raid5
have their own queues to control I/Os, where A write request may
cause several read requests and have to wait for their completions
before the actual write starts.

Hirokazu Takahashi.

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