On 12. apr. 2010, at 02.18, KaiGai Kohei wrote:
> Are you saying the data fragments are not necessary to be placed on the
> continuous region in physically?
> If so, it seems to me allocate() method needs to be revised to take an
> arguments to indicate the number of fragments (with their lengths) on
> allocation time?
>
> However, it seems to me the following description introduces all the
> needed region is allocated on the allocation time at once, then the data
> region will be segmented into several chunks.
>
Yes, the intention of the iovector is so that you can store the value for a key
as multiple fragments that would be concatenated onto the wire in the return
packet. This means that we could implement append / prepend really fast by just
adding another element to the internal iovector of an item.
We don't need to modify the allocate functions, because the core will call the
method to get the iovector for the item when it needs to access the value.
An important piece that is missing right now is the function the engine can
call to get the maximum number of fragments that the core supports. I should
get that in there as soon as possible.
>
> BTW, here is a point I missed it.
> Currently, do_add_delta() and do_store_item() with APPEND/PREPEND option
> assume the data field only contains the original data. So, if we inject
> extra attributes into the data region, it performs unexpectedly.
>
> For example, if we append {key="aaa", value="mokemoke"} after the
> {key="aaa", value="fugafuga"}, and both have "classified\0" as security
> label, it will concatenate them without eliminating extra attributes,
> like {key="aaa", value="fugafugaclassified\0mokemoke"}.
> ^^^^^^^^^^^^
> So, we need to inform secondary module where is the data segment and where
> is the extra attributes of items, even if the secondary module allocate
> data region in flat.
If you're storing meta-data inside the real data of an item, you would just
have to add your own implementation for append/prepend where you do the
operation on the items and then try to call a cas set operation to the engine.
Cheers,
Trond
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