Thank you for your explanation
and for the phy:device and file:device,which has better performance?
phy:device one for its direct path ,or the other one?
and I am also confused about how does the frontend I/O request remap to
the phyiscal device in the I/O path mode of "phy:device"? the blocks in
"phy:device" is continuous , or not? and in the "file:device",the blocks
are not continuous ,is it right?
Thanks in advance
Mark Williamson 写道:
I have read some documents and wiki about split driver in xen,and I am
confused about the I/O path ,in which a sys_read() pass through the domU
and dom0,does sys_read() in the domU pass through vfs and ,say ,ext3fs
in domU,and insert request into the requeest_queue of the
frontend-driver,is it right?
Sounds like you have the right idea. Requests get queued with the frontend
driver in terms of Linux structures. IO requests to satisfy these are then
placed into the shared memory ring so that the backend can find out what
we're asking for.
and then ,say domU sets up with a *.img file in the dom0, then what
does frontend and backend driver do?
does frontend transmit the request to the backend ,is it right?
Yes, the frontend does this by putting requests into the shared memory
ringbuffer which is also accessible by the backend. The frontend then sends
and event to the backend; this causes an interrupt in the backend so that it
knows it must check the shared memory.
and then what does backend driver do ? does backend transfer the
request to the phyiscal driver in the dom0 ,is it right?
Yes. The backend responds to the interrupt by checking the shared memory for
new requests, then it maps parts of the domUs memory so that dom0 will be
able to write data into it. Then it submits requests to the Linux block IO
subsystem to fill that memory with data. The Linux block IO system
eventually sends these requests to the device driver, to do the IO directly
into the mapped domU memory.
or does backend transfer the request into some read()operation ,and
submit it to the vfs and ,say,ext3fs in dom0, and do another relatively
complete io path in the dom0,is it right?
If you're just exporting a phy: device to the guest, then the block IO
requests go down to the block device driver for that device and are serviced
there. e.g. if I export IDE driver phy:/dev/hda to my guest, then the IDE
driver will satisfy the IO requests directly.
Requests go backend -> block layer -> real device driver
If you're using a file: device then you have to go through the filesystem
layer... So the IO requests go backend -> block layer -> loopback block
device -> ext3 -> block layer (again) -> real device driver
If you're using blktap then the requests take a trip via userspace before
getting submitted.
or if backend transfer the request to physical driver directly, how
does the backend deal with the request's virtual address ,and how does
backend manage bio buffer ,does physical driver and backend and frontend
share the bio buffer in some way, or what does xen deal with it ?
I hope what I've said clarifies things a bit.
Cheers,
Mark
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