Slideshow:
The 3 types of logging techniques
How is a log file maintained ?
How is a log file maintained ?
Because the buffer is notyet full, the buffer is not written to disk...
How is a log file maintained ?
Log writing rules (protocol)
The log writing rules ensures that the log records stored on disk can be use to recover a transaction
In order to ensure recoverability, we may need to force write a (non-full) log buffer to disk !
The log flush operation
The log flush operation is used to ensure that the necessary log records will survive a power failure (which will cause data stored in memory to be lossed)
Log writing with log flush
Log writing with log flush
Log writing with log flush
- Types of
logging techniques:
- Undo logging
- The log file contains log records that enable us to undo (= roll back) the changes made by an incomplete transaction
- Redo logging:
- The log file contains log records that enable us to redo (= roll forward) the changes made by a completed transaction
- Undo/redo logging:
- The log file contains
both undo and redo
log records and
enable us to
- undo (= roll back) the changes made by an incomplete transaction
- redo (= roll forward) the changes made by a completed transaction
- The log file contains
both undo and redo
log records and
enable us to
- Undo logging
- The log file consists of
2 parts:
Explanation:
- The older records of the
log file are
stored on
disk
- The newer log records of the log file are stored in memory
- The older records of the
log file are
stored on
disk
- When the transaction manager
writes a
new log record:
- the new log record
is appended to the
log records in the
memory buffer:
- the new log record
is appended to the
log records in the
memory buffer:
- When the memory buffer
becomes
full:
- The entire buffer
is written to disk:
- The buffer content is now
a disk block which
is appended to the
log file:
- The entire buffer
is written to disk:
- Very important fact:
- Each type of
log file has a number of:
- Log file writing rules
- The log file
writing rules specifies:
- An specifc ordering of write operations to the disk
The specifc disk write ordering must be obeyed (or else, the logging technique will not work properly) !!!
- Each type of
log file has a number of:
- Fact:
- Due to the
log writing rules in the
logging protocol,
we must
write the
log records to
disk
even when the
buffer is
not completely full:
- Due to the
log writing rules in the
logging protocol,
we must
write the
log records to
disk
even when the
buffer is
not completely full:
- The log flush operation:
- The log flush
operation will
force the
log records in
memory to be
written to disk:
- The log flush
operation will
force the
log records in
memory to be
written to disk:
- Comment:
- After a
log flush,
new log records can be
appended to
the log buffer in
memory:
- When the log buffer is
full:
We write the full buffer to disk and replace the partially filled data block:
- After a
log flush,
new log records can be
appended to
the log buffer in
memory: