Re: AW: (ITS#8777) [LMDB] Closing read cursor uses already freed transaction (MDB_VL32)

[hyc@symas.com]

markus@greenrobot.de wrote:
>> When using MDB_VL32 to access a 64bit DB on a 32bit CPU, the DB cannot fit
>> into a single mmap.
>> [...]
> 
> Long story short: I did not understand MDB_VL32 in the first place. I
> thought it would just enable 64 bit sizes.

Pointless, if you're still limited to 2GB. It would only eat up 4 extra bytes 
overhead in every internal integer.

>> Ideally, you should quit using obsolete 32bit CPUs.
> 
> Of course. Think about mobile and IoT however: Here, there are still 32 bit
> CPUs and it's not uncommon to even make 64 bit ARM CPUs operate in 32 bit
> mode to reduce binary size etc.
> 
> I'd like to propose a different approach to 64 bits on 32 bit CPUs;
> something like this:
> #ifdef MDB_SIZE64
> typedef uint64_t	mdb_size_t;
> ...
> 
> Why? MDB_VL32 adds complexity and it would take additional coding and
> testing efforts to get it production ready.

If you had used the API as intended, with txn_reset/renew as I originally 
suggested, there would be no problem.
> 
> I think MDB_SIZE64 might be a better trade-off. Of course this would only
> work with smaller DBs files (< 2GB/4GB). But I don't see a huge scenario for
> 32 bit CPUs being able to work with huge files anyway.

The reason MDB_VL32 exists at all is because of exactly this - people wanted 
to use 32 bit CPUs with 64 bit databases.

> The main reason I see
> to use 64 bits consistently across CPU architecture is binary compatibility.
> This is a big thing for Android (shipping apps with prefilled DBs). Also it
> enables getting DB files from 32 bit devices and open those on the desktop,
> etc.
> 
> What do you think?

Bad idea. MDB_VL32 provides *full* compatibility between 32bit and 64bit CPUs 
*regardless of filesize*. Your suggestion will only cause confusion, as people 
wonder why their DB dies after reaching 2GB in size. It offers none of the 
benefits of 64bit capability, while still paying all of the cost in terms of 
storage overhead.

-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
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