Buffer Management

1
Buffer Management

• Idea Minimize the amount of disk I/O by keeping
  likely-to-be-requested pages in memory (buffer
  frames)

John Sieg, UMass Lowell
2
Relationship to Other DBMS Modules

• 1. Request record.
• 2. Fix page.
• 3. Retrieve page into buffer.
• 4. Access memory by memory address.

John Sieg, UMass Lowell
3
Buffer Manager API

• mem_loc FIX(in page_id)
• fixed pages never victimized
• There can be multiple fixes on a single page.
• Address translation for page is done once, not
  many times.
• UNFIX(in page_id)
• issued soon after FIX
• Page may remain in buffer
• Flush(in list of pages)
• needed for atomic recovery

John Sieg, UMass Lowell
4
Implementing the Buffer Pool

• Header records can be statically allocated as an
  array, aligned with buffers.
• Why hash table?

John Sieg, UMass Lowell
5
FIX()
memory_address_type FIX (page_id_type page_id)
memory_address_type buffer_address
buffer_record_type buffer_record hash
page_id via buffer table hash function
traverse hash bucket, looking for page_id's
buffer record if (page identified by page_id
has a buffer record buffer_record)
buffer_address use_resident_copy(buffer_record)
else if free_list is non-empty
buffer_address use_free_buffer(page_id)
else if look_aside_list is non-empty
buffer_address victimize_a_look_aside_buffer(pag
e_id) else // There are no unfixed buffers.
handle overcommitment of buffer pool
return buffer_address
John Sieg, UMass Lowell
6
Buffer Pools Compared to File Caches

• Both services hidden from upper layers
• Both cache disk pages in memory and maintain
  look-aside lists
• File cache managers do not support FIX (why?)
• Replacement policies and prefetching techniques
  are more effective in buffer managers.
• File cache managers provide little support for
  recovery.

John Sieg, UMass Lowell
7
Replacement Policies

• Only UNFIXed pages (the look-aside list) are
  subject to replacement.
• Motivation Keep in buffers the pages most likely
  to be referenced
• Policies
• LRU (least recently UNFIXed)
• Clock
• newer policies that use DBMSs knowledge of
  reference string

John Sieg, UMass Lowell
8
Clock Replacement Policy
John Sieg, UMass Lowell
9
Why It Pays to be Clever
foreach tuple tr of relation r do foreach
tuple ts of relation s do output
concat(tr, ts)
Buffer pool
Relation r
Relation s
John Sieg, UMass Lowell