What's the meaning of DISK

[James]

CASE 1:
The following sentences were copied from the book "Administra tion
Guide Performance".

"If more pages have been written to disk, recovery of the database is
faster after a system crash because the database manager can rebuild
more of the buffer pool from disk instead of having to replay
transactions from the database log files."

My question is:
How to understand "rebuild buffer pool from disk"? Where is buffer
pool stored in disk?
CASE 2:
As we know, dirty pages contain data that has been changed but has not
yet been written to disk. After a changed page is written to disk, it
is considered clean again and remains in the buffer pool until its
space is needed for new pages. Page cleaner processes clean up the
buffer pool by writing dirty pages to disk. There are three types of
cleaning activities that can cause a page cleaner to take action and
remove a page or more from the buffer pool:
LSN Gap Triggers (e.g. Log Cleans)
Dirty Page Threshold Triggers (e.g. Threshold Cleans)
Dirty Page Steal Triggers (e.g. Victim Cleans)

My question is:
Except moving data into log files, where are the other dirty pages
moved to? If the are written to a temporary disk space, where is it?

Thanks in advance!!

James

[Haider Rizvi]

James <hu*****@gmail. comwrites:

CASE 1: The following sentences were copied from the book
"Administra tion Guide Performance".

"If more pages have been written to disk, recovery of the database
is faster after a system crash because the database manager can
rebuild more of the buffer pool from disk instead of having to
replay transactions from the database log files."

My question is: How to understand "rebuild buffer pool from disk"?
Where is buffer pool stored in disk?

Bufferpool pages, when first populated, are simple replica of pages on
disk. As transactions happen, these bp pages get "dirty". Before these
pages are dirtied, any changes are put in logs using write-ahead
logging (search the web for write-ahead logging or Aries for
details). These bp dirty pages are eventually written out to disk at
an opportune time, bringing the bp pages to a clean state thus making
them ready to be evicted if need be.

Hopefully this answers your 2nd question as well.

CASE 2: As we know, dirty pages contain data that has been changed
but has not yet been written to disk. After a changed page is
written to disk, it is considered clean again and remains in the
buffer pool until its space is needed for new pages. Page cleaner
processes clean up the buffer pool by writing dirty pages to
disk. There are three types of cleaning activities that can cause
a page cleaner to take action and remove a page or more from the
buffer pool: LSN Gap Triggers (e.g. Log Cleans) Dirty Page
Threshold Triggers (e.g. Threshold Cleans) Dirty Page Steal
Triggers (e.g. Victim Cleans)

My question is: Except moving data into log files, where are the
other dirty pages moved to? If the are written to a temporary disk
space, where is it?

Thanks in advance!!

James

--
Haider

[Ian]

James wrote:

CASE 1:
The following sentences were copied from the book "Administra tion
Guide Performance".

"If more pages have been written to disk, recovery of the database is
faster after a system crash because the database manager can rebuild
more of the buffer pool from disk instead of having to replay
transactions from the database log files."

My question is:
How to understand "rebuild buffer pool from disk"? Where is buffer
pool stored in disk?

It's not. Dirty bufferpool pages are written back to disk by the page
cleaners.

After a database crash, it's often the case that there were dirty pages
in the bufferpool that hadn't been written to disk. So DB2 needs a way
to make sure that the tablespace pages on disk get this information,
and it does this keeping track of the LSN of the oldest dirty page in
the bufferpool(s).

DB2 keeps track of 2 things:

LSN of the oldest uncommitted transaction (called LowTranLSN)
LSN of the oldest dirty page in the bufferpool (called MinBufLSN)

When doing crash recovery, DB2 determines uses the minimum of these 2
LSNs to as the starting point for replaying the log files.

So, the statement you're asking about is basically saying that if
MinBufLSN is always greater than LowTranLSN, your crash recovery will be
faster -- because DB2 will spend less time rebuilding the dirty pages
that were in the bufferpool at the time of the crash. (i.e., there were
fewer dirty pages).

CASE 2:
As we know, dirty pages contain data that has been changed but has not
yet been written to disk. After a changed page is written to disk, it
is considered clean again and remains in the buffer pool until its
space is needed for new pages. Page cleaner processes clean up the
buffer pool by writing dirty pages to disk. There are three types of
cleaning activities that can cause a page cleaner to take action and
remove a page or more from the buffer pool:
LSN Gap Triggers (e.g. Log Cleans)
Dirty Page Threshold Triggers (e.g. Threshold Cleans)
Dirty Page Steal Triggers (e.g. Victim Cleans)

My question is:
Except moving data into log files, where are the other dirty pages
moved to? If the are written to a temporary disk space, where is it?

They are written back to the tablespaces where they came from.