On 5/10/13 1:32 PM, Simon Riggs wrote:
> The timing
> window between the write and the sync is negligible and yet I/O would
> need to occur in that window and also be out of order from the order
> of the write, which is unlikely because an I/O elevator would either
> not touch the order of writes at all, or would want to maintain
> sequential order to avoid head movement, which is what we want. I
> guess we should add here "...with disks, maybe not with SSDs".
It's not really safe to make any assumptions about I/O elevators.
Reordering gets done from the perspective of the last item written.
When the previous write was at the logical end of the disk, it can just
as easily re-order a queue of writes in the complete reverse order they
were issued in.
The only way you can ever get a useful guarantee is when an fsync
returns completion. Writes can effectively go out in a completely
random order until that point. All you can rely on is throwing up a
stop sign that says "tell me when all of them are done". In between
those, you have no idea of the ordering.
--
Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
