RE: [PoC] Non-volatile WAL buffer

Hello Robert,

I think our concerns are roughly classified into two:

(1) Performance
(2) Consistency

And your "different concern" is rather into (2), I think.

I'm also worried about it, but I have no good answer for now. I suppose mmap(flags|=MAP_SHARED) called by multiple backend processes for the same file works consistently for both PMEM and non-PMEM devices. However, I have not found any evidence such as specification documents yet.

I also made a tiny program calling memcpy() and msync() on the same mmap()-ed file but mutually distinct address range in parallel, and found that there was no corrupted data. However, that result does not ensure any consistency I'm worried about.　　I could give it up if there *were* corrupted data...

So I will go to (1) first. I will test the way Heikki told us to answer whether the cost of mmap() and munmap() per WAL segment, etc, is reasonable or not. If it really is, then I will go to (2).

Best regards,
Takashi

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Takashi Menjo <takashi(dot)menjou(dot)vg(at)hco(dot)ntt(dot)co(dot)jp>
NTT Software Innovation Center