confused about block device behaviour

confused about block device behaviour

[lord latex]

hi

i've got written a block device driver for the 2.4.x 
kernel and it seems to work. but something looks 
strange to me. i've go a very simple application that 
does nothing more then open the block device, read 
1024 byte and close the device. when i run this app. 
serveral times my do_request function gets called 
every time. why? i expect this block beeing buffered 
in the buffer cache. what do i don't see, or what is 
possibly wrong with my block device?

thanks

Re: confused about block device behaviour

[Andrew Morton]

lord latex wrote:
> 
> hi
> 
> i've got written a block device driver for the 2.4.x
> kernel and it seems to work. but something looks
> strange to me. i've go a very simple application that
> does nothing more then open the block device, read
> 1024 byte and close the device. when i run this app.
> serveral times my do_request function gets called
> every time. why? i expect this block beeing buffered
> in the buffer cache. what do i don't see, or what is
> possibly wrong with my block device?
> 

The kernel invalidates the device's cache on the final close.
If you hold the device open in a different process:

	sleep 1000 < /dev/foo

while you run the test, you'll see that the underlying device's
contents are indeed cached.

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