checkpoint internals of postgresql. buffer cache basics in postgresql checkpoint definition...

CheckPoint Internals of PostgreSQL

Buffer Cache Basics in PostgreSQL

Checkpoint Definition

Checkpoint Implementation in PostgreSQL

Advantages & Disadvantages of PostgreSQL Implementation

Buffer Cache Basics in Oracle

Checkpoint Implementation in Oracle

Advantages & Disadvantages of Oracle Implementation

Improvements in PostgreSQL


• The buffer cache component structures are:

• Buffers –

Each buffer may hold an image of one data block at any one

time

• Buffer headers –

Store metadata about contents of the buffers –

Act as cache management structures

• Buffer Pool

– A freelist of buffers is maintained, from which a buffer is

assigned to a new page request.


• Hash Table

– Hash table is to get faster the shared buffer for a page if it

is already in shared buffer pool.

– Hash value is calculated as – Oid of tablespace,

database, relation; Filetype, blocknumber.

– Hash table has 16 partitions which will ensure

that 16 concurrent access to hash table will be allowed.


• Usage count in buffer header tracks the usage of buffer based on which it is decided whether to flush particular buffer when no more free buffers are available.

• Once freelist buffers are finished, backend runs the clock sweep algorithm to find a suitable buffer for reuse.

• If that page is dirty, it is written out to disk. This can cause the backend trying to allocate that buffer to block as it waits for that write I/O to complete.

• Clock sweep also marks nextvictim buffer which will be used to traverse the list next time backend wants a buffer for a new page request.

• A buffer becomes dirty at a particular LSN (Log sequence number) which is a point in WAL file.

• Dirty page images written by the background writer or checkpoint makes sure that corresponding part of WAL file is written flushing page.

15

53

42

93

156

Search For Blocks Already In Buffer Pool

Request For block with hash key 42

42




Block not found in existing buffers. Search for a free buffer for block in buffer freelist.

93

156

Search for buffer in Freelist Of Buffers

bid 1 uc 2

bid 2 uc 3

bid 3 uc 1

bid 4 uc 5

bid 5uc 2

bid 6 uc 0

bid 7 uc 0

bid 6 uc 0

bid 7 uc 0

Buffer Pool

FreeList

Get Buffer

bid 7uc 1

bid 6uc 1

No buffer in freelist; run clock sweep on buffer pool

Clock Sweep for buffer in Buffer Pool

rcnt 1 uc 2

rcnt 0 uc 3

rcnt 0 uc 0

rcnt 0 uc 5

rcnt 2uc 2

Buffer Pool

Get Buffer

rcnt 0uc 0

rcnt 0uc 1

Check if victim buffer doesn’t have ref count 0; then move to next buffer

If ref count is 0, then if Usage Cnt > 0 then decrement it and move to next buffer.

rcnt 0 uc 2

If refcnt and usage cnt is 0 then return buff.

rcnt 0 uc 0rcnt 1 uc 1

rcnt 0 uc 4

rcnt 0uc 0

rcnt 0uc 0rcnt 1uc 1

Buffer Cache I/O in PostgreSQL

• Buffer contents are flushed at following events - When backend doesn’t find free buffers in freelist. - Page writer at predefined interval - Checkpoint - due to DDL like Alter Table or Tablespace all buffers having pages corresponding to that table will be flushed.

• For operations which needs many pages access but don’t reuse previous page, a different buffer access strategy is used.

• It creates a ring of buffers and then only buffers from the ring will be used. This limits the usage of buffers for heavy operations.

• Operations for which this strategy is used are as follows: Vacuum, Bulk Read (SeqScan), BulkWrite (insert into … select * ..)


• A checkpoint is a point in the transaction log sequence at which all

data files have been updated to reflect the information in the log.

• At checkpoint time, all dirty data pages are flushed to disk and a

special point is noted to determine the point in the log from which it

should start the REDO operation during CRASH recovery.

• Checkpoint occurs at following events

• When predefined log segment gets full

• Checkpoint timeout interval

• CHECKPOINT command


• A background checkpoint process monitors events which can cause

checkpoint and performs required actions.

• To avoid flooding the I/O system with a burst of page writes, page

writer will intermittently flush dirty pages and writing dirty buffers

during a checkpoint can spread over a period of time.

Background Writer

• There is a separate server process called the background writer,

who does 2 activities : -

Page writing, the functionality is to issue writes of some “dirty”

shared buffers. -

Checkpoint, whose function is to write all “dirty” buffers and

other shared memory buffers (Commit Log buffers,

Subtrasaction pages).

It also remove old WAL files which are now not required.

• It will automatically dispatch a checkpoint after a certain amount of

time has elapsed since the last one

Background Writer

• It can be signaled to perform requested checkpoints as well.

• Backends signal the bgwriter when they fill WAL segments to

perform checkpoint.

• If the bgwriter exits unexpectedly, the postmaster treats that the

same as a backend crash: shared memory may be corrupted, so

remaining backends should be killed by SIGQUIT and then a

recovery cycle started.

Page Writing

• Page writing is to ensure that sessions seldom or never have to wait

for a write to occur, because the background writer will do it. However,

regular backends (sessions) are still empowered to issue writes if the

bgwriter fails to maintain enough clean shared buffers.

• In general due to Page writer there is a net overall increase in I/O load,

because a repeatedly-dirtied page might otherwise be written only

once per checkpoint interval, but the background writer might write it

several times in the same interval.

• ‘bgwriter_delay’ specifies the delay between activity rounds for the

background writer. In each round the writer issues writes for some

number of dirty buffers

• In each round, no more than ‘bgwriter_lru_maxpages’ buffers will be

written by the background writer.

Page Writing Contd..

• The number of dirty buffers written in each round is based on the number of new buffers that have been needed by server processes during recent rounds.

• It will also consider the number of reusable buffers from previous bgwriter run which are accumulated based on extra pages it has flushed than expected.

• It will start from nextvictim buffer, which will be useful for backends as chances of getting them useful buffers is high.

• It will flush the blocks one by one. There is no concept of grouping buffers for flushing.

• Even in cases where there's been little or no buffer allocation activity, we want to make a small amount of progress through the buffer cache so that as many reusable buffers as possible are clean after an idle period.

Checkpoint Implementation

• Internally checkpoint is done at shutdown or at end of recovery.• To avoid flooding the I/O system with a burst of page writes, writing

dirty buffers during a checkpoint is spread over a period of time. • That period is controlled by checkpoint_completion_target, which

is given as a fraction of the checkpoint interval. • With the default value of 0.5, PostgreSQL can be expected to

complete each checkpoint in about half the time before the next checkpoint starts.

• Top level Checkpoint Design• In the start of process, it marks the current WAL location as the

Redo Replay location incase of crash recovery.

Checkpoint Implementation Contd..• Before flushing data, we must wait for any transactions that are currently

in their commit critical sections. - If an xact inserted its commit record into XLOG just before the REDO point, then a crash restart from the REDO point would not replay that record.- so this checkpoint flushing better include the xact's update of pg_clog.

• Flush all the shared memory data to disk.

• Write out all dirty buffers in the pool - Loop over all buffers, and mark the ones that need to be written with BM_CHECKPOINT_NEEDED. - This allows us to write only those pages that were dirty when the checkpoint began. - Loop over all buffers again, and write the ones marked with BM_CHECKPOINT_NEEDED. In this loop, we start at the clock sweep point since we might as well dump soon-to-be-recycled buffers first.

Checkpoint Implementation Contd..

• Each write (either by backend, pagewrite, checkpoint) will put a request in Fsync request queue.

• After writing pages in checkpoint, it calls Fsync filewise so that all blocks of that file will be written to disk together. This will reduce random I/O.

• It writes Checkpoint Redo record which contains information like Redo Replay start pos., nextXid, nextOid etc.

• Delete old log files (those no longer needed even for previous checkpoint.

Advantages/Disadvantages of PG Implementation

• Advantages

1. To reduce I/O throttle during checkpoint bgwriter intermittently

flushes pages; also during checkpoint operation it sleeps based on

checkpoint progress.

2. It limits the usage of buffers for heavy operations such as

Sequence Scan, Vacuum.

Advantages/Disadvantages of PG Implementation

• Disadvantages

1. Currently bgwriter never puts the buffer in freelist even if it can

identify that the buffer is reusable.

2. Once buffers are over in freelist, it has to always traverse serially

the buffer pool.

3. Bgwriter_delay is at fixed interval rather than activity based.

4. Currently there is no protection for pg_control file, only it is

controlled as the structure is currently 512 bytes which is one sector

write.

Buffer Cache Basics In Oracle• The buffer cache component structures are:

• Buffers – Each buffer may hold an image of one data block at any one time

• Buffer headers – Store metadata about contents of the buffers – Act as cache management structures

• Buffer pools – Collection of buffers used for the same purpose and managed accordingly

• Working set– All or part of a buffer pool– Assigned to a DBWn process

Buffer Management

• Cached buffers managed by doubly linked lists:• REPL –

Buffers containing block images being used• REPL-AUX

– Buffers ready to be used for I/O or CR build• WRITE and CKPT-Q –

Dirty Buffers requiring I/O. Two different queues are maintained for write buffers as checkpoint queue ensure buffers will be written in order or lowest RBA.

• WRITE-AUX – Dirty Buffers with I/O in progress

• Touch count is used to decide the initial insertion location in theREPL chain

• Buffers are moved from main to auxiliary list by DBWn processes to avoid unnecessary scans

Hash Buckets

• Hash value of each block calculated from– Data Block Address (DBA)– Block Class

• Number of hash buckets dependent on number of buffers in cache e.g.

• Each hash bucket contains– Cache Buffers Chains latch– Pointer to array of double linked lists

# buffers 500 6000

# hash buckets 64 1024

Hash Buckets

BH BH

BH

BH

# hash chains

cachebufferschainlatch

Buffer Headers• Each buffer header describes contents of one buffer

• All buffers accessed via buffer header

• Buffer header contains pointers to– Buffer– Cache Buffers Chains latch

• Buffer header includes double linked lists for– Cache Buffers Chain list– Replacement list– Users list– Waiters list

Replacement List

• In Oracle 8.1.5 and above a mid-point insertion algorithm is used• Buffer cache has a hot end and a cold end• Buffers are inserted at mid-point• Mid-point is head of cold end• Starts at hot end - moves down cache• Maximum mid-point determined by _db_percent_hot_default• Default value is 50%

Head of Hot End

Head of Cold End

Hot End Cold EndReplacement List

Touch Count

• Each buffer header maintains– touch count– timestamp

• Touch count represents number of 3 second intervals in which buffer has been accessed since– buffer last read into cache– touch count last reset

• Each time buffer is accessed– if timestamp more than 3 seconds ago; increment touch

count– set timestamp to current time

Touch Count

• When buffer reaches tail of cold end– If touch count >= 2 then – buffer is moved to hot end– Otherwise used as next free buffer

• Hot criteria determined by– _db_aging_hot_criteria– default value is 2 touches

Head of Cold End

Head of Hot End

Single-Block Reads

92

0

34

3

72

4

52

1

71

2

66

0

49

0

42

1

45

2

52

1

71

2

66

0

42

1

11

1

52

1

71

2

11

1

42

1

42

2

71

0

92

0

34

3

72

4

45

2

11

1

52

1

42

2

33

1

45

2

11

1

42

2

33

1

34

4

92

0

34

4

72

4

45

2

11

1

42

0

33

1

71

0

87

1

87

1

72

4

33

1

45

2

Read Block 42

Get first available buffer from cold endUpdate buffer contentsInsert buffer at head of cold end

Read Block 11

Get first available buffer from cold endUpdate buffer contentsInsert buffer at head of cold end

Read Block 42

Update touch count for block 42

Read Block 33

Move block 71 to head of hot endSet touch count on block 71 to zeroGet first available bufferfrom cold endUpdate buffer contentsInsert buffer at head of coldend

Read Block 34

Update touch countfor block 34

Read Block 87

Move block 42 to headof hot endSet touch counton block 42 to zeroGet first available bufferfrom cold endUpdate buffer contentsInsert buffer at head of coldend

STOP

Block Number

Touch Count

Checkpoint In Oracle

• The point of Oracle checkpoints is to synchronize all datafiles, some datafiles or some objects to a point in time for consistency, performance and recoverability purposes.

• The Database Writer Process

The Database Writer (DBWR) process is responsible for writing data from the RAM buffer cache to the database datafiles on disk. The default is to have one database writer process, but large databases can have multiple DBWR processes.

• The Checkpoint Process

The checkpoint process is responsible for updating file headers in the database datafiles. A checkpoint occurs when Oracle moves new or updated blocks (called dirty blocks) from the RAM buffer cache to the database datafiles.

Dbwriter (DBWn) Process

• If an Oracle user process searches the threshold limit of buffers without finding a free buffer, the process stops searching the LRU list and signals the DBW0 background process to write some of the dirty buffers to disk.

• To allow buffer replacements to occur, it is necessary to write cold dirty buffers.Such writes are referred to as aging writes and are performed by DBWn processes.

• The DBWn processes work to write enough cold buffers to guarantee a uniform supply of free buffers for replacement purposes.

• Do forever { Scan lru list; Scan checkpoint queue; Accumulate batch of buffers to write; Issue writes; Wait for completion of writes; }

Multiple Dbwriters

• A mechanism for increasing write throughput.

• Buffer cache is partitioned between dbwriters by working sets.

• Each DBWn process scans its own assigned working sets.

Set 0 Set 1 Set 2 Set 3

DBW0 DBW1

Writes to Datafiles

Why Multiple Dbwriters

• Reasons to consider multiple dbwriters include high wait times for: - Free buffer waits (Cache not cleaned fast enough) - Logfile switch: Checkpoint incomplete (checkpoint not advanced out of previous log, blocking redo generation)• Also consider multiple dbwriters if a single dbwriter consumes 100%

of a CPU.• It is useful when the transaction rates are high or when the buffer

cache size is so large that a single DBWn process cannot keep up with the load.

• Free buffer waits due to slow writes to saturated disks will not go away with multiple dbwriters.

• A good indication is time spent on “db file sequential read” (foreground reads)

If this time is high then bottleneck is disk not the number of dbwriters.

Types Of Checkpoint

• Several types of checkpoint exist • Full Checkpoint

• Thread Checkpoint

• File Checkpoint

• Object “Checkpoint”

• Parallel Query Checkpoint

• Incremental Checkpoint

• Log Switch Checkpoint

Incremental Checkpoint

• The objective was to make the algorithm scalable to very large buffer caches, and to facilitate frequent checkpointing for fast crash recovery.

• Writes the contents of “some” dirty buffers to the database from CKPT-Q

• Checkpoint RBA updated in SGA

• Every 3 seconds CKPT calculates the checkpoint target RBA based on:

▫ The most current RBA

▫ log_checkpoint_timeout

▫ log_checkpoint_interval

▫ fast_start_mttr_target

▫ fast_start_io_target

▫ 90% of the size of the smallest online redo log file

• LGWR flushes the redo buffer up to the target RBA

Incremental Checkpoint

• DBWR writes dirty buffers from CKPT-Q up to the target RBA

▫ Moves batches of buffers to WRITE-AUX

▫ Does the write

▫ Moves the buffers to REPL-AUX. They can be safely reused

• CKPT updates the control file with the checkpoint progress

• Since it is performed continuously, the value of the CKPT RBA will be much closer to the tail of the log than the RBA of a “conventional” checkpoint, thus limiting the amount of recovery needed.

Buffer Ckpt Queue

• Buffer Checkpoint Queue (BCQ)– Ordered by RBA of first change (first-dirty or low RBA)– Buffer is linked into BCQ when first dirtied– Writes in BCQ order advance thread checkpoint

(Thread Checkpoint)c0

b2b11 b4b3

c1 c2 c3 c4

Buffers

Redo Thread

Buffer Ckpt Queue

Each write in BCQ order advances the checkpoint

c0 c2 c3 c4

b2b11 b3 b4

(Write b1)

b2

c1 c3 c4c2

b3 b4

(Write b2)

c3 c4

b3 b4

(Write b3)

c3 c4

b4

c1

c2

Full Checkpoint

• Writes block images to the database for all dirty buffers from all instances

• Caused by:

– Alter system checkpoint [global]

– Alter database begin backup

– Alter database close

– Shutdown

• Controlfile and Datafile headers are updated

– CHECKPOINT_CHANGE#


• Advantages1. It uses multiple lists to avoid unnecessary scanning of free

buffers.

2. It can scale to having many buffers in memory

3. It has checksum mechanism for data blocks to protect from disk damage issues.

4. Flexibility of multiple writters to avoid bottlenecks and maximize I/O usage

5. Disadvantages

6. If the block gets corrupted due to partial write, it has to use backup copy and incremental logs to recover database.


1. Can improve buffer management so that it can scale to large number of buffers and reduce buffer scanning.

2. Double-write using temp file for page corruption during checkpoint.

3. Checksum in pages to ensure if page is corrupted in disk due to disk torn or any other similar reason.

4. Bgwriter to write based on buffer usage, not only at fixed intervals.

5. Have the mechanism to recover pg_control file from disk corruption.

6. Can have Incremental checkpoint strategy to improve recovery time and make database scale to large number of buffers.

Thank You

checkpoint internals of postgresql. buffer cache basics in postgresql checkpoint definition...

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