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how are the bits in ASSM calculated? [message #677545]

Wed, 25 September 2019 04:48

Flodul
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Registered: September 2019

Junior Member

Does anyone know the algorithm ORacle uses for calculating the bits that indicate if a block has free space for new rows with automatic segment space management?
Thanks in advance.

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Re: how are the bits in ASSM calculated? [message #677546 is a reply to message #677545]

Wed, 25 September 2019 05:18

Michel Cadot
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This is recorded in bitmaps located in the segment header.

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Re: how are the bits in ASSM calculated? [message #677548 is a reply to message #677546]

Wed, 25 September 2019 06:46

Flodul
Messages: 2
Registered: September 2019

Junior Member

I didn't want to know wherer they are stored but as I said how the bits are calculated.
Oracke of course doesn't clear a bit if the last byte in a block is used but rather on a dynamic threshold.
But how is this thresshold calculated?
Maybe there's a patent on this.

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Re: how are the bits in ASSM calculated? [message #677549 is a reply to message #677548]

Wed, 25 September 2019 07:34

Michel Cadot
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What is the purpose of this question?
What do you want to do with this information?
Maybe this will help to know what answer you want.

And, sure, there are patents on how the bitmaps are used.

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Re: how are the bits in ASSM calculated? [message #677553 is a reply to message #677548]

Thu, 26 September 2019 01:35

John Watson
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Registered: January 2010
Location: Global Village

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There are four bitmaps per segment which track block usage in 25% bands. So every block that has been used has an entry on one of the bitmaps, depending on how full it is. Is that what you are asking?

(One minor point of purely academic interest, Michel: I'm sure I remember reading that the bitmaps are not necessarily in the segment header any more, they can be distribute through the segment. This was a change introduced at some point for some obscure performance reason).

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Re: how are the bits in ASSM calculated? [message #677554 is a reply to message #677553]

Thu, 26 September 2019 01:57

Michel Cadot
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I meant by segment header all metadata about the segment, you are right, they may be spread into several and non contiguous extends (header Smile

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