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Why do the KDBs take up more space in memory than on disk?

johns175

The documentation on Checkpointing says this: "When using multiple gateways for an engine, a KDB file normally consumes a much larger amount of RAM than the size of the KDB file on disk, as all transactions are maintained both in RAM and on disk." I understand that the KDB in memory in this case would be larger than the KDB memory in the case where there is only a single gateway for an engine, because in the case of a single gateway the KDB in memory would not maintain all the transactions. But why would the KDB in memory in the case of multiple gateways consume a much larger amount of RAM than the size of the KDB on disk if all transactions are maintained both in RAM and on disk. Wouldn't these transactions take up the same amount of space in memory as on disk?...

OriginalPostID-48198

OriginalPostID-48198

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    Appian Employee
    John, perhaps the phrasing in the documentation could be better, but in general, data size in memory is much larger than on disk. If you add the fact that in multiple gateway architecture you keep the transaction log in memory (for performance of gw synchronization) the discrepancy between kdb on disk and in memory grows even larger. I do not exactly know why the data on disk is smaller, but I assume it has to do with the fact that not only data but the code manipulating that data is also loaded into memory. I will follow up with our engineers to find out if my assumption is correct.
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    Mike, thank you for your response. I think that your explanation is probably correct. There are both programs and data in memory and only data on disk. And when the transactions are also stored in memory for gw synchronization, then more memory is taken up. However, the explanation in the documentation does not make any sense. I think it is very important to have clear and accurate documentation if you want people to learn how to use your software.

    I really appreciate it that you have taken the time to respond to me. I will probably be asking you some more questions as I work more with Appian.
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