Re: Design proposal: fsync absorb linear slider

On 07/26/2013 11:42 AM, Greg Smith wrote:
> On 7/25/13 6:02 PM, didier wrote:
>> It was surely already discussed but why isn't postresql writing
>> sequentially its cache in a temporary file?
>
> If you do that, reads of the data will have to traverse that temporary
> file to assemble their data. You'll make every later reader pay the
> random I/O penalty that's being avoided right now. Checkpoints are
> already postponing these random writes as long as possible. You have
> to take care of them eventually though.
>
Well, SSD disks do it in the way proposed by didier (AFAIK), by putting
"random"
fs pages on one large disk page and having an extra index layer for
resolving
random-to-sequential ordering.

I would not dismiss the idea without more tests and discussion.

We could have a system where "checkpoint" does sequential writes of dirty
wal buffers to alternating synced holding files (a "checkpoint log" :) )
and only background writer does random writes with no forced sync at all

--
Hannu Krosing
PostgreSQL Consultant
Performance, Scalability and High Availability
2ndQuadrant Nordic OÜ