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Re: MDB v2: Replace meta pages with "meta position" word
Howard Chu writes:
>Hallvard Breien Furuseth wrote:
>>> One thing about ACID - IIRC there are some potential data races which my
>>> single-word meta info in the header will fix. E.g. if the user does ^Z
>>> at an unfortunate time, and with WRITEMAP updating the meta page while a
>>> txn is reading it.
>> Reproducible by sending frequent SIGSTOPs and SIGCONTs, with mdb.c
>> patched to verify (memcmp or checksum) the meta info after copying.
>> Happens even with 1 MDB process.
>> (Re comments elsewhere: Selecting a meta page is atomic, but
>> copying/writing it is not.)
> True, but writing the meta page doesn't need to be atomic - it won't be
> considered current until its txnid is updated. Until then, its txnid will be
> less than the other page's anyway.
I did test it. Maybe the signal caught a reader in the middle of
copying a meta page, then several write transactions changed both metas.
I see WRITEMAP can only be relevant if something reoders some accesses,
which I see no particular reason to happen in this case. Or at least
not compared to reordering meta page vs. me_txns->mti_txnid access.