Re: Reducing ClogControlLock contention

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От Simon Riggs
Тема Re: Reducing ClogControlLock contention
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Msg-id CANP8+jKUfWNz3FQS_2o19bV7vESO+v_WmBqkVrRoHPr1pTs5Kw@mail.gmail.com
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Ответ на Re: Reducing ClogControlLock contention  (Andres Freund <andres@anarazel.de>)
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On 22 August 2015 at 15:14, Andres Freund <andres@anarazel.de> wrote:
 
TransactionIdSetPageStatus() calls TransactionIdSetStatusBit(), which
writes an 8 byte variable (the lsn). That's not safe.

This point was the main sticking point here.

It turns out that we don't need to store the LSN (8 bytes). 

WAL is fsynced every time we finish writing a file, so we only actually need to store the byte position within each file, so no more than 16MB. That fits within a 4 byte value, so can be written atomically. 

So I have a valid way forward for this approach. Cool.

--
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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