Re: about fsync in CLOG buffer write

Andres,

Thanks for the reply!

I will try the ForwardFsyncRequest-like approach.

在 15-9-2 下午8:32， "Andres Freund" <andres(at)anarazel(dot)de> 写入:

>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.
>
>You could try increasing the max (32) in CLOGShmemBuffers() further.
>
>> ctl->do_fsync is true for CLOG. Question is, could we just disable
>>fsync
>> for CLOG buffer write out here? Is it safe to do so? I understand a
>> checkpoint will calls SimpleLruFlush to flush all CLOG buffer at once,
>>and
>> the fsync call here (for buffer write out) is not necessary.
>
>No, that'd not be safe. The reason we fsync in SlruPhysicalWritePage()
>is that the flush during checkpoint won't necessarily touch those files
>at all (as they've been replaced with other buffers in memory).
>
>This could be optimized though - it should be possible to delay the
>fsync for slru files that have been evicted from memory till
>checkpoint. Using something like ForwardFsyncRequest() except that it
>obviously has to be usable for other files than normal relfilenodes.
>
>Greetings,
>
>Andres Freund