block read ahead algorithm

harit agarwal · 05-20-2010, 12:16 AM

algorithm:breada
input:file system block number for immediate read
file system block number for asynchronous read
output:buffer containing data for immediate read
{
if(first block not in cache)
{
get buffer for first block
if(buffer data not valid)
initiate disk read
}
if(second block not in cache)
{
get buffer for second block)
if(buffer data valid) //line 1
release buffer
else
initiate disk read //line 2
}
if(first block was originally in cache) //line 3
{
read first block
return buffer
}
sleep(event first buffer contains valid data)
return buffer
}

here is an algorithm for block read algorithm. i have problem in
line 1: if buffer data is valid why is it releasing the buffer?
line 2: if buffer data valid why is it initiating disk read. it should have read directly from buffer?
line 3:it should be the first condition as if it is there in cache then it should return it without delay?

so please clear my doubts in these statements.....

Sergei Steshenko · 05-20-2010, 12:43 AM

Quote:

Originally Posted by harit agarwal

algorithm:breada
input:file system block number for immediate read
file system block number for asynchronous read
output:buffer containing data for immediate read
{
if(first block not in cache)
{
get buffer for first block
if(buffer data not valid)
initiate disk read
}
if(second block not in cache)
{
get buffer for second block)
if(buffer data valid) //line 1
release buffer
else
initiate disk read //line 2
}
if(first block was originally in cache) //line 3
{
read first block
return buffer
}
sleep(event first buffer contains valid data)
return buffer
}

here is an algorithm for block read algorithm. i have problem in
line 1: if buffer data is valid why is it releasing the buffer?
line 2: if buffer data valid why is it initiating disk read. it should have read directly from buffer?
line 3:it should be the first condition as if it is there in cache then it should return it without delay?

so please clear my doubts in these statements.....

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murugesan · 05-20-2010, 01:31 AM

Kindly read the book:
Bach M.J. The design of the Unix operating system related to chapter 3:

Quote:

http://www.booklists.narod.ru/S_Soft...C__486s_.3.htm

Links given here for each chapter:

Quote:

http://murugesan.webnode.com/books/

or available at beginning of each chapter.

Quote:

Top / Software manuals / [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

I prefer to use the book instead of using the url mentioned here as it does not contain the diagrams mentioned in each chapter.

Refer "4.7 ALLOCATION OF DISK BLOCKS"
Refer how OS uses buffer cache.

harit agarwal · 05-20-2010, 03:18 AM

i read this algorithm from the same book but still i have some issues after reading it (as i already posted in my first post).....so please clarify those doubts....

murugesan · 05-20-2010, 04:32 AM

Quote:

Originally Posted by harit agarwal

line 1: if buffer data is valid why is it releasing the buffer?

This requires information to read on "brelse" algorithm (provided you have gone through 5 steps mentioned in "the kernel may follow in getblk".)

Quote:

Originally Posted by harit agarwal

line 2: if buffer data valid why is it initiating disk read. it should have read directly from buffer?

Kernel will go to initiate disk read when the buffer data is invalid.

Quote:

Originally Posted by harit agarwal

line 3:it should be the first condition as if it is there in cache then it should return it without delay?

This is because the following statement is given in the book:
"If the second block is not in the buffer cache, the kernel instructs the disk driver to read it ASYNCHRONOUSLY."

harit agarwal · 05-20-2010, 05:34 AM

thanks i got it.