[bind10-dev] shared memory data source: file-based (mmap) vs memory-only

[Shane Kerr]

Jinmei,

On Wednesday, 2013-02-27 21:05:01 -0800, 
JINMEI Tatuya / 神明達哉 <jinmei at isc.org> wrote:
> Before detailing the feature and breaking it into development tasks,
> I have two big questions to discuss.  This is one of them.
> 
> There are roughly two choices:
> 1. use a file (or files) containing the memory image and map it to the
>    memory
> 2. build everything in-memory
 

> As far as I can see and based on my quick experiments, both classes
> will meet our minimal need.  The major difference is the use of
> concrete files.
> 
> I personally prefer the file-based approach, because this can work as
> a persistent on-disk copy that enables nearly-zero (hot start case) or
> a few-to-10-ish seconds (cold start case, with some initial work) of
> start up time.  The major disadvantage is management overhead of the
> files and the risk of having corrupt on-disk images.
> 
> Do others have opinions/preferences on this?

No preference on my side.

> On a related note: I remember there was a mention of the use of
> copy-on-write when updating the current version of the shared memory
> image.  As far as I can see, we can't benefit from it, at least not if
> we use the above Boost utility classes.  We'll first need to make a
> complete copy of the current image, either on disk or in-memory and
> then updated the copied version.

I looked a little under the hood, and it seems like the use case of
efficiently cloning a shared-memory segment doesn't have any answer,
outside of fork(). :(

Cheers,

--
Shane