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videlord
Beginner
Joined: 09 Dec 2004
Posts: 147
Topics: 19
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 Posted: Thu Nov 24, 2005 10:20 pm Post subject: Record length limitation |
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Why the record length of each record cannot be greater than 4056 bytes for 4K tabke space?
Every data page has 22-byte header, where is the other 18 bytes?
The following is copied from DB2 Administrator Guide:
Furthermore, the page size of the table space in which the table is defined limits the record length. If the table space is 4 KB, the record length of each record cannot be greater than 4056 bytes. Because of the 8-byte overhead for each record, the sum of column lengths cannot be greater than 4048 bytes (4056 minus the 8-byte overhead for a record). |
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kolusu
Site Admin
Joined: 26 Nov 2002
Posts: 12378
Topics: 75
Location: San Jose
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| Posted: Sat Nov 26, 2005 8:59 am Post subject: |
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videlord,
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Every data page has 22-byte header, where is the other 18 bytes?
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Videlord,
Each page set is composed of several types of pages:
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1. header page
2. space map pages
3. dictionary pages
4. data pages
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The header page contains control information used by DB2 to manage and maintain the table space. For example, the OBID and DBID (internal object and database identifiers used by DB2) of the table space and database are maintained here, as well as information on logging. Each linear page set has one header page; every partition of a partitioned page set has its own header page. The header page is the first page of a VSAM data set.
Space map pages contain information pertaining to the amount of free space available on pages in a page set. A space map page outlines the space available for a range of pages.
Dictionary pages are used for table spaces that specify COMPRESS YES. Information is stored in the dictionary pages to help DB2 control compression and decompression. The dictionary pages are stored after the header page and first space map page, but before any data pages.
Data pages contain the user data for the table space or index page set. The layout of a data page depends on whether it is an index data page or a table space data page.
Hope this helps...
Cheers
kolusu
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Kolusu
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videlord
Beginner
Joined: 09 Dec 2004
Posts: 147
Topics: 19
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| Posted: Sun Nov 27, 2005 10:23 pm Post subject: |
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kolusu,
Thanks for your explain about the page type in DB2.
But my question is about the data page (for table data). For a certain DB2 data page, why there is a such limitatoin for table record length: 4056 for 4K, 8138 for 8k ... Why it's not 4074 (4096 - 22) bytes? |
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Manas Biswal
Intermediate
Joined: 29 Nov 2002
Posts: 382
Topics: 27
Location: Chennai, India
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| Posted: Mon Nov 28, 2005 10:31 am Post subject: |
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videlord,
Your question set me thinking (and searching - ). I wonder why I have not wondered about that questions earlier.
It seems that DB2 allows 4074 bytes of data for a 4K tablespace if there are more than 1 records in a single page. But if a "single record" is there on a page, then it allows a maximum of 4056 bytes for that record.
Basically, to answer your question, DB2 does theoritically allow 4074 bytes in a 4K tablespace for data (although there are a lot of practical impediments to using 4074 bytes; for example the default PCTFREE is 5%), but thats only when there are more than 1 records in a tablespace.
As to why, it does not allow 4074 bytes for data for a single record in a 4K tablespace, I am still searching -.
HTH...Regards,
Manas
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