RE: what is light scan and look aside??
All of these 'advanced' questions. Are you retreading from some other vendor? You should consider a course. Light Scan: Normally when you read a table, you go through the buffer pool. That is your row is retrieved from disk, stuck into a buffer, an LRU entry is made to track that buffer. If you are reading a big table, you will fill the buffer and ruin everybody else's day, and have to worry about all of the buffer management. A light scan uses it's own set of private buffer pools - so it leaves the main buffer pool alone. There is minimal buffer management for these pools. It can run about 2-4x faster than a normal buffered read. Number of light scan buffers allocated is a function of your Read ahead settings - max them out. To get a light scan: - the table being read has to be larger than your BUFFSIZE Note - optimizer won't know this unless you update stats - the isolation mode has to be effectively dirty read (bunch of ways to do this) - you can't have varchars in the table (not true for XPS) Look Aside: The problem with a light scan is that you are reading disk, what happens if the data you wanted to read was already in a buffer - and potentially altered? Enter the look aside. Your light scan continues, but pages that are in the buffer are threaded into your results. cheers j. > -----Original Message----- > From: sudhakar [mailto:k_sudhak@rediffmail.com] > Sent: Saturday, February 03, 2001 3:30 AM > To: informix-list@iiug.org > Subject: what is light scan and look aside?? > > > hello, > > please explain briefly what is Light scan and Look aside. > > bye.. > sudhakar > > -- > Posted via CNET Help.com > http://www.help.com/ >
If you ever take the informix test on www.brainbench.com you will understand the questions :) He will do better on it now :) Will In article <95p9mi$oej$1@news.xmission.com>, "Parker, Jack" <JParker@engage.com> wrote: > > > All of these 'advanced' questions. Are you retreading from some other > vendor? You should consider a course. > > Light Scan: > > Normally when you read a table, you go through the buffer pool. > That is your row is retrieved from disk, stuck into a buffer, an LRU entry > is made to track that buffer. If you are reading a big table, you will fill > the buffer and ruin everybody else's day, and have to worry about all of the > buffer management. A light scan uses it's own set of private buffer pools - > so it leaves the main buffer pool alone. There is minimal buffer management > for these pools. It can run about 2-4x faster than a normal buffered read. > Number of light scan buffers allocated is a function of your Read ahead > settings - max them out. > > To get a light scan: > - the table being read has to be larger than your BUFFSIZE > Note - optimizer won't know this unless you update stats > - the isolation mode has to be effectively dirty read (bunch of ways to do > this) > - you can't have varchars in the table (not true for XPS) > > Look Aside: > > The problem with a light scan is that you are reading disk, what > happens if the data you wanted to read was already in a buffer - and > potentially altered? Enter the look aside. Your light scan continues, but > pages that are in the buffer are threaded into your results. > > cheers > j. > > > -----Original Message----- > > From: sudhakar [mailto:k_sudhak@rediffmail.com] > > Sent: Saturday, February 03, 2001 3:30 AM > > To: informix-list@iiug.org > > Subject: what is light scan and look aside?? > > > > > > hello, > > > > please explain briefly what is Light scan and Look aside. > > > > bye.. > > sudhakar > > > > -- > > Posted via CNET Help.com > > http://www.help.com/ > > > Sent via Deja.com http://www.deja.com/