Re: Experimental patch for inter-page delay in VACUUM

Bruce Momjian wrote:

> Jan Wieck wrote:
>> Zeugswetter Andreas SB SD wrote:
>> 
>> >> > One problem with O_SYNC would be, that the OS does not group writes any 
>> >> > more. So the code would need to eighter do it's own sorting and grouping
>> >> > (256k) or use aio, or you won't be able to get the maximum out of the disks.
>> >> 
>> >> Or just run multiple writer processes, which I believe is Oracle's
>> >> solution.
>> > 
>> > That does not help, since for O_SYNC the OS'es (those I know) do not group those 
>> > writes together. Oracle allows more than one writer to busy more than one disk(subsystem) and circumvent other per
processlimitations (mainly on platforms without AIO). 
 
>> 
>> Yes, I think the best way would be to let the background process write a 
>> bunch of pages, then fsync() the files written to. If one tends to have 
>> many dirty buffers to the same file, this will group them together and 
>> the OS can optimize that. If one really has completely random access, 
>> then there is nothing to group.
> 
> Agreed.  This might force enough stuff out to disk the checkpoint/sync()
> would be OK.  Jan, have you tested this?
> 

As said, not using fsync() but sync() at that place. This only makes a 
real difference when you're not running PostgreSQL on a dedicated 
server. And yes, it really works well.


Jan

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