Information Technology Reference
In-Depth Information
database disk
log disk
n
1
n
2
n
3
n
4
n
5
update on
q
p
Page -LSN=
page
q
Page -LSN=
page
update on
p
n
2
n
1
checkpoint
update on
p
commit
fix page
flush page
flush
log
n
6
n
7
n
8
n
9
update on q
page
p
Page -LSN=
page
q
Page -LSN=
update on
p
n
9
n
8
update on
q
update on
p
database buffer
log buffer
Fig. 2.3
Database state after a sequence of logged actions
checkpoint
,ora
fuzzy checkpoint
, in which only some of the modified pages are
flushed. In a minimalistic fuzzy checkpoint, no database pages are flushed, but
only some information about currently buffered pages are written to the log. This
information includes page identifiers with
LSN
s indicating positions in the log after
which there might be updates that are missing from the disk versions of pages;
in restart recovery, this information is used to determine those pages whose disk
versions might have some updates missing (see Chap.
4
).
Example 2.3
Figure
2.3
shows the states of two pages after a sequence of actions
logged with
LSN
s n
1
to n
9
. The updates up to
LSN
n
2
are present in the disk versions
of pages p and q because the pages have been flushed from the buffer onto disk when
taking the checkpoint logged with
LSN
n
3
. Before the pages were flushed, following
the write-ahead logging protocol, the log up to n
2
was flushed from the log buffer
onto the log disk. Next time the log was flushed when a transaction was committed
at
LSN
n
5
. After that, active transactions updated pages p and q, generating the log
records with
LSN
s n
6
to n
9
appended to the log buffer.
The current versions of pages p and q are in the database buffer. The
P
AGE
-
LSN
values stamped in the page headers indicate that the current version of page p
contains all the updates logged with
LSN
s n
2
, n
4
, n
7
,andn
9
, while the disk version
of p does not contain the three last updates, and that the current version of page q
