Re: about fsync in CLOG buffer write

On September 12, 2015 5:18:28 PM PDT, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>On Wed, Sep 2, 2015 at 5:32 AM, Andres Freund <andres(at)anarazel(dot)de>
>wrote:
>
>> On 2015-09-10 19:39:59 +0800, 张广舟(明虚) wrote:
>> > We found there is a fsync call when CLOG buffer
>> > is written out in SlruPhysicalWritePage(). It is often called when
>a
>> backend
>> > needs to check transaction status with SimpleLruReadPage().
>>
>> That's when there's not enough buffers available some other, and your
>> case dirty, needs to be written out.
>>
>
>Why bother to find a place to store the page in shared memory at all?
>If
>we just want to read it, and it isn't already in shared memory, then
>why
>not just ask the kernel for the specific byte we need? The byte we
>want to
>read can't differ between shared memory and kernel, because it doesn't
>exist in shared memory.

I doubt that'd help - the next access would be more expensive, and we'd need to have a more complex locking regime. These pages aren't necessarily read only at that point.

Andres

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