Recap of Feb 25: Physical Storage Media

1
Recap of Feb 25 Physical Storage Media

• Issues are speed, cost, reliability
• Media types
• Primary storage (volatile) Cache, Main Memory
• Secondary or On-line storage (non-volatile)
  Flash Memory, Mag Disk
  
• Tertiary or Off-line storage (non-volatile)
  Optical Storage, Tape Storage
  
• Mag disk issues
• definitions sector, track, cylinder
• disk controllers, multiple disks
• disk performance measures (seek time, rotational
  latency, data transfer rate, MTTF)
  
• Now we start with Optimization of Disk-Block
  Access
  

2
Optimization of Disk-Block Access Motivation

• Requests for disk I/O are generated both by the
  file system and by the virtual memory manager
  
• Each request specifies the address on the disk to
  be referenced in the form of a block number
  
• a block is a contiguous sequence of sectors from
  a single track on one platter
  
• block sizes range from 512 bytes to several K (4
  -- 16K is typical)
  
• smaller blocks mean more transfers from disk
  larger blocks makes for more wasted space due to
  partially filled blocks
  
• block is the standard unit of data transfer
  between disk to main memory
  
• Since disk access speed is much slower than main
  memory access, methods for optimizing disk-block
  access are important

3
Optimization of Disk-Block Access Methods

• Disk-arm Scheduling requests for several blocks
  may be speeded up by requesting them in the order
  they will pass under the head.
  
• If the blocks are on different cylinders, it is
  advantageous to ask for them in an order that
  minimizes disk-arm movement
  
• Elevator algorithm -- move the disk arm in one
  direction until all requests from that direction
  are satisfied, then reverse and repeat
  
• Sequential access is 1-2 orders of magnitude
  faster random access is about 2 orders of
  magnitude slower
  

4
Optimization of Disk-Block Access Methods

• Non-volatile write buffers
• store written data in a RAM buffer rather than on
  disk
  
• write the buffer whenever it becomes full or when
  no other disk requests are pending
  
• buffer must be non-volatile to protect from power
  failure
  
• called non-volatile random-access memory
  (NV-RAM)
  
• typically implemented with battery-backed-up RAM
• dramatic speedup on writes with a
  reasonable-sized buffer write latency essentially
  disappears
  
• why cant we do the same for reads? (hints ESP,
  clustering)
  

5
Optimization of Disk-Block Access Methods

• File organization (Clustering) reduce access
  time by organizing blocks on disk in a way that
  corresponds closely to the way we expect them to
  be accessed
• sequential files should be kept organized
  sequentially
  
• hierarchical files should be organized with
  mothers next to daughters
  
• for joining tables (relations) put the joining
  tuples next to each other
  
• over time fragmentation can become an issue
• restoration of disk structure (copy and rewrite,
  reordered) controls fragmentation
  

6
Optimization of Disk-Block Access Methods

• Log-based file system
• does not update in-place, rather writes updates
  to a log disk
  
• essentially, a disk functioning as a non-volatile
  RAM write buffer
  
• all access in the log disk is sequential,
  eliminating seek time
  
• eventually updates must be propogated to the
  original blocks
  
• as with NV-RAM write buffers, this can occur at a
  time when no disk requests are pending
  
• the updates can be ordered to minimize arm
  movement
  
• this can generate a high degree of fragmentation
  on files that require constant updates
  
• fragmentation increases seek time for sequential
  reading of files
  

7
Storage Access (11.5)

• Basic concepts (some already familiar)
• block-based. A block is a contiguous sequence of
  sectors from a single track blocks are units of
  both storage allocation and data transfer
  
• a file is a sequence of records stored in
  fixed-size blocks (pages) on the disk
  
• each block (page) has a unique address called
  BID
  
• optimization is done by reducing I/O, seek time,
  etc.
  
• database systems seek to minimize the number of
  block transfers between the disk and memory. We
  can reduce the number of disk accesses by keeping
  as many blocks as possible in main memory.
• Buffer - portion of main memory used to store
  copies of disk blocks
  
• buffer manager - subsystem responsible for
  allocating buffer space in main memory and
  handling block transfer between buffer and disk

8
Buffer Management

• The buffer pool is the part of the main memory
  alocated for temporarily storing disk blocks read
  from disk and made available to the CPU
  
• The buffer manager is the subsystem responsible
  for the allocation and the management of the
  buffer space (transparent to users)
  
• On a process (user) request for a block (page)
  the buffer manager
  
• checks to see if the page is already in the
  buffer pool
  
• if so, passes the address to the process
• if not, it loads the page from disk and then
  passes the address to the process
  
• loading a page might require clearing (writing
  out) a page to make space
  
• Very similar to the way virtual memory managers
  work, although it can do a lot better (why?)

9
Buffer Replacement Strategies

• Most operating systems use a LRU replacement
  scheme. In database environments, MRU is better
  for some common operations (e.g., join)
  
• LRU strategy replace the least recently used
  block
  
• MRU strategy replace the most recently used
  block
  
• Sometimes it is useful to fasten or pin blocks to
  keep them available during an operation and not
  let the replacement strategy touch them
  
• pinned block is thus a block that is not allowed
  to be written back to disk
  
• There are situations where it is necessary to
  write back a block to disk even though the buffer
  space it occupies is not yet needed. This write
  is called the forced output of a block useful in
  recovery situations
• Toss-immediate strategy free the space occupied
  by a block as soon as the final tuple of that
  block has been processed

10
Buffer Replacement Strategies

• Most recently used (MRU) strategy system must
  pin the block currently being processed. After
  the final tuple of that block has been processed
  the block is unpinned and becomes the most
  recently used block. This is essentially
  toss-immediate with pinning, and works very
  well with joins.
• The buffer manager can often use other
  information (design or statistical) to predict
  the probability that a request will reference a
  particular page
• e.g., the data dictionary is frequently accessed
  -- keep the data dictionary blocks in main memory
  buffer
  
• if several pages are available for overwrite
  choose the one that has the lowest number of
  recent access requests to replace

11
Buffer Management (cont)

• Existing OS affect DBMS operations by
• read ahead, write behind
• wrong replacement strategies
• Unix is not good for DBMS to run on top
• Most commercial systems implement their own I/O
  on a raw disk partition
  
• Variations of buffer allocation
• common buffer pool for all relations
• separate buffer pool for each relation
• as above but with relations borrowing space from
  each other
  
• prioritized buffers for very frequently accessed
  blocks, e.g. data dictionary

12
Buffer Management (cont)

• For each buffer the manager keeps the following
• which disk and which block it is in
• whether the block is dirty (has been modified) or
  not (why?)
  
• information for the replacement strategy
• last time block was accessed
• whether it is pinned
• possible statistical information (access
  frequency etc.)
  

13
Buffer Management and Disk-block Access
Optimization (end)

• Disk-block access methods must take care of some
  information within each block, as well as
  information about each block
  
• allocate records (tuples) within blocks
• support record addressing by address and by
  value
  
• support auxiliary (secondary indexing) file
  structures for more efficient processing
  
• These concerns are linked in to our next topic
• file organization.