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[Qemu-discuss] Writing a QEMU block driver

From: Sandeep Joshi
Subject: [Qemu-discuss] Writing a QEMU block driver
Date: Sat, 18 Oct 2014 08:42:44 +0530


I'd like to write a QEMU block driver which forwards IO requests to a custom-built storage cluster. 

I have seen Jeff Cody's presentation and also browsed the source code for sheepdog, nbd and gluster in the "block" directory and had a few questions.

1) What is the difference between bdrv_open and bdrv_file_open function pointers in the BlockDriver ?

2) Is it possible to implement only a protocol driver without a format driver (the distinction that Jeff made in his presentation above) ? Do they work together or does one have to implement both for a particular file format and storage destination?   I'd like to support all image formats (qemu, raw, etc) without having to reimplement the logic for each.  

3) The control flow for creating a file starts with the image format driver and later invokes the protocol driver.

    --> bdrv_create_file
          --> bdrv_find_protocol(filename)
          --> bdrv_create
                ---> Protocol_driver->bdrv_create()

Is this the case for all functions?   Does the read/write first flow through the image format driver before getting passed down to the protocol driver (possibly via some coroutine invoked from the block layer or virtio-blk ) ?  Can someone give me a hint as to how I can trace the control flow ?


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