 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
| HomeDiscussion Groups Database Servers DB2InformixIngresMS SQLOraclePervasive.SQLPostgreSQLProgressSybaseDesktop Databases FileMakerFoxProMS AccessParadoxGeneral General DB TopicsDatabase TheoryRelated Topics Java Development.NET DevelopmentVB DevelopmentMore Topics ... |
Database Forum / DB2 Topics / May 2007What's the meaning of DISK
CASE 1: The following sentences were copied from the book "Administration 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 > CASE 1: The following sentences were copied from the book > "Administration Guide Performance". [quoted text clipped - 6 lines] > 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 [quoted text clipped - 14 lines] > > James  SignatureHaider > CASE 1: > The following sentences were copied from the book "Administration [quoted text clipped - 8 lines] > 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 [quoted text clipped - 11 lines] > 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. > > CASE 1: > > The following sentences were copied from the book "Administration [quoted text clipped - 49 lines] > > They are written back to the tablespaces where they came from. Prefetcher loads pages from disk to buffer pool. All these pages are clean before they are modified. When a page is modified, but this change has not been committed by transaction, this page becomes a dirty page. This dirty page is still in buffer pool and page cleaner writes this dirty page into log. If DB2 use write-ahead to record all changes into log, this page was written into log before it was changed. (Write to log before and after changes??) If the transaction is committed, this dirty page is written into the table which it comes from. If the transaction is rollback, where will the dirty page go? I dont' think it will be written into the table space where it comes from. Do I misunderstand the whole process? I would really appreciate if someone can show me a whole process. When database crash, where will the dirty page go if this dirty has not been committed? How does DB2 rebuild buffer pool? I think DB2 will write all buffer pool into a temporary file on hard disk when database crash. Is my thought right? I would really appreciate if someone give a correct point. Thanks in advance! > > CASE 1: > > The following sentences were copied from the book "Administration [quoted text clipped - 49 lines] > > They are written back to the tablespaces where they came from. Prefetcher loads pages from disk to buffer pool. All these pages are clean before they are modified. When a page is modified, but this change has not been committed by transaction, this page becomes a dirty page. This dirty page is still in buffer pool and page cleaner writes this dirty page into log. If DB2 use write-ahead to record all changes into log, this page was written into log before it was changed. (Write to log before and after changes??) If the transaction is committed, this dirty page is written into the table which it comes from. If the transaction is rollback, where will the dirty page go? I dont' think it will be written into the table space where it comes from. Do I misunderstand the whole process? I would really appreciate if someone can show me a whole process. When database crash, where will the dirty page go if this dirty has not been committed? How does DB2 rebuild buffer pool? I think DB2 will write all buffer pool into a temporary file on hard disk when database crash. Is my thought right? I would really appreciate if someone give a correct point. Thanks in advance! > > CASE 1: > > The following sentences were copied from the book "Administration [quoted text clipped - 49 lines] > > They are written back to the tablespaces where they came from. Prefetcher loads pages from disk to buffer pool. All these pages are clean before they are modified. When a page is modified, but this change has not been committed by transaction, this page becomes a dirty page. This dirty page is still in buffer pool and page cleaner writes this dirty page into log. If DB2 use write-ahead to record all changes into log, this page was written into log before it was changed. (Write to log before and after changes??) If the transaction is committed, this dirty page is written into the table which it comes from. If the transaction is rollback, where will the dirty page go? I dont' think it will be written into the table space where it comes from. Do I misunderstand the whole process? I would really appreciate if someone can show me a whole process. When database crash, where will the dirty page go if this dirty has not been committed? How does DB2 rebuild buffer pool? I think DB2 will write all buffer pool into a temporary file on hard disk when database crash. Is my thought right? I would really appreciate if someone give a correct point. Thanks in advance! > > CASE 1: > > The following sentences were copied from the book "Administration [quoted text clipped - 49 lines] > > They are written back to the tablespaces where they came from. Prefetcher loads pages from disk to buffer pool. All these pages are clean before they are modified. When a page is modified, but this change has not been committed by transaction, this page becomes a dirty page. This dirty page is still in buffer pool and page cleaner writes this dirty page into log. If DB2 use write-ahead to record all changes into log, this page was written into log before it was changed. (Write to log before and after changes??) If the transaction is committed, this dirty page is written into the table which it comes from. If the transaction is rollback, where will the dirty page go? I dont' think it will be written into the table space where it comes from. Do I misunderstand the whole process? I would really appreciate if someone can show me a whole process. When database crash, where will the dirty page go if this dirty has not been committed? How does DB2 rebuild buffer pool? I think DB2 will write all buffer pool into a temporary file on hard disk when database crash. Is my thought right? I would really appreciate if someone give a correct point. Thanks in advance! |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.