On 6/8/13 5:20 PM, Kevin Grittner wrote:
I'll believe that all of that is true, but I think there's another reason. With multiple layers of write cache (PostgreSQL shared_buffers, OS cache, controller or SSD cache) I think there's a tendency for an "avalanche" effect. Dirty pages stick to cache at each level like snow on the side of a mountain, accumulating over time. When it finally breaks loose at the top, it causes more from lower levels to break free as it passes.
I explained this once as being like a tower of leaky buckets where each one drips into the one below. Buckets draining out of the bottom at one rate, and new water comes in at another. You can add water much faster than it drains, for a while. But once one of the buckets fills you've got a serious mess.
I think the "sudden onset" of problems from write saturation contributes to the complaints.
It's also important to realize that vacuum itself doesn't even do the writes in many cases. If you have a large shared_buffers value, it wanders off making things dirty without any concern for what's going to disk. When the next checkpoint shows up is when pressure increases at the top.
The way this discussion has wandered off has nicely confirmed I was right to try and avoid going into this area again :(
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