Re: Load Distributed Checkpoints test results

On Fri, 15 Jun 2007 22:28:34 +0200, Gregory Maxwell   
wrote:

> On 6/15/07, Gregory Stark  wrote:
>> While in theory spreading out the writes could have a detrimental  
>> effect I
>> think we should wait until we see actual numbers. I have a pretty strong
>> suspicion that the effect would be pretty minimal. We're still doing  
>> the same
>> amount of i/o total, just with a slightly less chance for the elevator
>> algorithm to optimize the pattern.
>
> ..and the sort patching suggests that the OS's elevator isn't doing a
> great job for large flushes in any case. I wouldn't be shocked to see
> load distributed checkpoints cause an unconditional improvement since
> they may do better at avoiding the huge burst behavior that is
> overrunning the OS elevator in any case.
...also consider that if someone uses RAID5, sorting the writes may  
produce more full-stripe writes, which don't need the read-then-write  
RAID5 performance killer...