Access block level storage via kernel

Question

How to access block level storage via the kernel (w/o using scsi libraries)?

My intent is to implement a block level storage protocol over network for learning purpose, almost the same way SCSI works. Requests will be generated by initiator and sent to target (both userspace program) which makes call to kernel module and returns the data using TCP protocol to initiator.

So far, I have managed to build a simple "Hello" module and run it (I am new at kernel programming), but unable to proceed with block access.

After searching a lot, I found struct buffer_head * bread(int dev,int block) in linux/fs.h, but the compiler throws error.

 error: implicit declaration of function ‘bread’

Please help, also feel free to advice on starting with kernel programming.
Thank you!

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bread as used in old kernels.

Looking into struct request *blk_get_request(struct request_queue *, int, gfp_t); in linux/blkdev.h

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Accessing the block device has to be accomplished via kernel.

Answer 1

Not a kernel developer, but a few comments:

• The implicit declaration error means that the definition you've found somehow isn't in scope when you call the function. Maybe it's hidden in an #ifdef or maybe you forgot to include linux/fs.h somehow.

• As far as advice on linux kernel programming, you might want to check out kernelnewbies.org.

• There have been various books written on kernel programming, but be aware that the details in the kernel change very rapidly. Most of the concepts in the older books will still be valid, but at least some of the details in some areas will have changed.

• Finally, you might have to brave the linux kernel mailing list. It's rather intimidating, I'm sorry to say, so try to have your questions well thought out before you post them.

• A block level storage protocol is itself a fair bit of work. Perhaps you want to get the protocol in place in user space first, with the target doing direct access to, eg, /dev/sdc before diving into the kernel.

As I read your question more closely, it appears your main interest is in the storage protocol aspect of this project. If so, why do you need to modify the kernel. If you have a locally attached disk, say /dev/sdX on the target, then you can do something like this from user space:

fd = open("/dev/sdX", O_RDWR);
pwrite(fd, buf, len, offset);
pread(fd, buf, len, offset);

So, unless you're specifically interested in playing around inside the kernel, I don't think you need to do any kernel module to do a basic storage protocol between user processes.