Xen devel - Sep 2008 - questions about the block backend/frontend driver

Hi, Everyone,

I am trying to understand the code of the block backend/frontend driver. I
konw the blkback and blkfront communicate with each other through event
channel and buffer ring. But there are some questions I could not
understand.

1. When dom0 receive one disk request from domU1 and another disk request
from domU2 simultaneously, how these two disk requests are pushed into Linux
Kernel I/O Scheduler? How the Xen sort them before pushing them into the
Linux Kernel I/O Scheduler?

2. Is there a disk request queue in the Xen to hold these disk requests, so
that the Xen could only push the disk requests in the queue into the linux
kernel I/O Scheduler?

I would appreciate it very much if anyone could give me some help.

Yuming


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Nobody answer these questions? I want to know if I could implement the I/O
disk scheduling in Xen and then I could not modify the corresponding
code(I/O Scheduling algorithms) of Linux Kernel?

Yuming

2008/9/8 Yuming fang <fangyuming.leo@gmail.com>
>  Hi, Everyone,
>
> I am trying to understand the code of the block backend/frontend driver. I
> konw the blkback and blkfront communicate with each other through event
> channel and buffer ring. But there are some questions I could not
> understand.
>
> 1. When dom0 receive one disk request from domU1 and another disk request
> from domU2 simultaneously, how these two disk requests are pushed into
Linux
> Kernel I/O Scheduler? How the Xen sort them before pushing them into the
> Linux Kernel I/O Scheduler?
>
> 2. Is there a disk request queue in the Xen to hold these disk requests, so
> that the Xen could only push the disk requests in the queue into the linux
> kernel I/O Scheduler?
>
> I would appreciate it very much if anyone could give me some help.
>
> Yuming
>
>

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On Tue, Sep 09, 2008 at 08:43:07PM +0800, Yuming fang
wrote:
> Nobody answer these questions? I want to know if I could implement the I/O
> disk scheduling in Xen and then I could not modify the corresponding
> code(I/O Scheduling algorithms) of Linux Kernel?
I would recommend you get ''blktrace'' programs and run them
under Dom0 to
get a practical answer to your question.

> 
> Yuming
> 
> 2008/9/8 Yuming fang <fangyuming.leo@gmail.com>
> 
> >  Hi, Everyone,
> >
> > I am trying to understand the code of the block backend/frontend
driver. I
> > konw the blkback and blkfront communicate with each other through
event
> > channel and buffer ring. But there are some questions I could not
> > understand.
> >
> > 1. When dom0 receive one disk request from domU1 and another disk
request
> > from domU2 simultaneously, how these two disk requests are pushed into
Linux
> > Kernel I/O Scheduler? How the Xen sort them before pushing them into
the
> > Linux Kernel I/O Scheduler?
That depends on which elevator you have. Xen does not sort them, just issues
a bio request.
> >
> > 2. Is there a disk request queue in the Xen to hold these disk
requests, so
> > that the Xen could only push the disk requests in the queue into the
linux
> > kernel I/O Scheduler?
No. It uses the block devices queue to submit requests,
> >
> > I would appreciate it very much if anyone could give me some help.
> >
> > Yuming
> >
> >
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

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Thank you very much for your reply.
2008/9/9 Konrad Rzeszutek <konrad@virtualiron.com>
> On Tue, Sep 09, 2008 at 08:43:07PM +0800, Yuming fang wrote:
> > Nobody answer these questions? I want to know if I could implement the
> I/O
> > disk scheduling in Xen and then I could not modify the corresponding
> > code(I/O Scheduling algorithms) of Linux Kernel?
>
> I would recommend you get ''blktrace'' programs and run
them under Dom0 to
> get a practical answer to your question.
>

I run the blktrace programs and could get the detailed information of disk
I/O read/write requests. However, the blktrace is a tool which provides
detailed information about request queue operations to the user space.


>
> Yuming
>
> 2008/9/8 Yuming fang <fangyuming.leo@gmail.com>
>
> >  Hi, Everyone,
> >
> > I am trying to understand the code of the block backend/frontend
driver.
I
> > konw the blkback and blkfront communicate with each other through
event
> > channel and buffer ring. But there are some questions I could not
> > understand.
> >
> > 1. When dom0 receive one disk request from domU1 and another disk
request
> > from domU2 simultaneously, how these two disk requests are pushed into
Linux
> > Kernel I/O Scheduler? How the Xen sort them before pushing them into
the
> > Linux Kernel I/O Scheduler?
That depends on which elevator you have. Xen does not sort them, just
issues
> a bio request.
>


Yeah, after your explanation, I read the code of blkback.c and understand
some about it. In the Xen3.3 version, I find there is a
\linux-2.6.18-xen-3.3.0\block\ directory, which includes the elevator.c,
Linux I/O Scheduling algorithm files(cfq-iosched.c, deadline-iosched.c and
so on).  Do these files decide the disk I/O Scheduling Algorithm of the
Linux-xen0? And if I want to add a different disk I/O Scheduling algorithm
in Xen3.3, could I add it in this
directory(linux-2.6.18-xen-3.3.0\linux-2.6.18-xen-3.3.0\block\)?


>
> > >
> > > 2. Is there a disk request queue in the Xen to hold these disk
> requests, so
> > > that the Xen could only push the disk requests in the queue into
the
> linux
> > > kernel I/O Scheduler?
>
> No. It uses the block devices queue to submit requests,
>


I think it uses the functions of unplug_queue(blkif_t *blkif) and
plug_queue(blkif_t *blkif, struct bio *bio)  (in
\linux-2.6.18-xen-3.3.0\drivers\xen\blkback\blkback,.c) to process the
requests. Is my explanation right?


Thanks
Yuming


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