1. CS 2606 Project 2 Clarifications

2. Clarifications Output specs Write-through versus write-back buffers
The remove operation and the buffer pool
Note: all of these clarifications are consistent with the project specs and the sample files posted

3. Output specs insert command print command remove command
insert #: record length = # + # = #, stored at position #
print command
print #
<record contents>
remove command
remove #: at position #, record length = #
Note: if the insert command involves a remove, only the insert operation produces output

4. Output specs buffers command freelist command Notes:
buffers: b1 b2 b3 …
freelist command
freelist: p1 s1, p2 s2, p3 s3, …
Notes:
buffer indices and free list positions begin at 0
buffer list starts with LRU block, ends with MRU block
free list is sorted by position

5. Write-through vs Write-back buffers
Write-through buffers:
insert command always results in a physical disk write
Write-back buffers:
physical disk write occurs only when necessary during a page fault (disk writes are minimized)
Write-through buffers are easier to implement, you get extra-credit (5%) if you implement a write-back buffer
In both cases, the insert command affects the buffer pool (blocks involved in the operation become the most recently used blocks)

6. Example
Suppose size of buffer pool = 2, and all records are 200 bytes each
Consider the sequence:
command write-through write-back
insert 1 write fblock 1
insert 2 write fblock 1
insert 3 write fblocks 1,2
print 1
print 2
print 3
insert 4 write fblock 2
insert 5 write fblock 2
insert 6 write fblocks 2,3 write fblock 1
no disk operations

7. Example, continued
After the last command, buffer pool contains fblocks 2, 3
If we continue with the following operations:
command write-through write-back …
insert 6 write fblocks 2,3 write fblock 1
print 1 fblk 1 in, fblk 2 out fblk 1 in, write fblk 2
print 2
print 3 fblk 2 in, fblk 3 out fblk 2 in, write fblk 3
print 4
print 5
print 6 fblk 3 in, fblk 1 out fblk 3 in, no write *
* write not necessary because block 1 not dirty

8. The remove operation
Question: does the remove operation require a buffer pool update or a disk operation?
Maybe, depending on how complex we want this project to be
For simplicity, you may assume that remove is strictly a memory manager operation (refer to the previous slide set about this project); no buffer pool update will result from a remove
Important: you need to deal with obtaining the size of the removed record (keep a table, or retrieve the 4 bytes from the file, without using the buffer pool)