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In a Xen setup, IO accesses from guest VMs go through a privileged domain called dom0 that is just a modified Linux kernel which has calls from and to the XEN hypervisor. For block IO, they have a split driver model whose front-end is in the guest VM and the backend of the driver in the domain0. The backend just creates a 'bio' structure and invokes submit_bio() as in traditional linux block driver code.

My goal here is to check if there is any problem in the data written to disk(lost data, silently corrupted writes, misdirected writes, etc). So I need to read the data that was written to disk and compare it with a on-cache copy of data (this is a common disk function called 'read after write'). My question is, is it not possible to invoke __bread() from my backend driver level ? The kernel crashes when __bread is invoked.. Could anyone understand the reason for this ? Also, if this ain't possible, what other ways are out there to read a specific block of data from disk at the driver's bottom-half ?

Can I intercept and clone the bio structure of the writes, and change the operation as read in my new bio and invoke submit_bio() again ? I did that, but the sector number in the bio structure that is returned by the completion callback of submit_bio() is some random value and not the ones I sent..

Thanks.

Sankaran Sivathanu
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If this were my task, I'd try first writing a new scheduling algorithm. Start by copying cfq or deadline or noop or as scheduling code and start working on it from there to self-submit read commands after accepting write requests. noop would probably be the easiest one to modify to read immediately after write, and propagate errors upwards, but I can't imagine the performance would be very good. But, if you use one of the other schedulers as base, it would probably be much more difficult to signal an error immediately after the write -- perhaps a few seconds would have elapsed before reads were scheduled again -- so it would really only be useful as a diagnostic after the fact, and not something that could benefit applications directly.

sarnold
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