Re: [PATCH v2] kvm tools: Add QCOW level2 caching support

[Ingo Molnar <mingo@elte.hu>]

* Prasad Joshi <prasadjoshi124@gmail.com> wrote:

> I am not sure how to induce the delay you mentioned. [...]

In the simplest version just add:

	if (debug__io_delay)
		udelay(1000);

to the code that does a read() from the disk image. This will 
introduce a 1 msec delay - that should be plenty enough to simulate 
most effects of IO delays.

Later on you could complicate it with some disk geometry 
approximation:

	delay = read_size_in_kb;	/* 1 usec per KB read */
	delay += seek_distance_in_kb;   /* 1 usec per KB disk-head movement */
	udelay(delay);

Where 'read_size_in_kb' is the number of bytes read in KB, while 
seek_distance_in_kb measures the position of the the last byte read 
by the previous read() call to the first byte of the current read().

( Also, instead of copying the disk image to /dev/shm you could add a 
  debug switch to mmap() and mlock() it directly. Assuming there's enough
  RAM in the box. )

But i'd strongly suggest to keep the initial version simple.

Thanks,

	Ingo