Re: XLog vs SSD [Was: Re: random write in xlog?]

On Wed, Dec 8, 2010 at 12:15 PM, James Cloos <cloos(at)jhcloos(dot)com> wrote:
>>>>>> "JJ" == Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
>
> JJ> Anyway, the writes are logically sequentially, but not physically.
> JJ> If I remember correctly, it always writes out full blocks, even if
> JJ> the last part of the block has not yet been filled with new data.
> JJ> When the remainder gets filled, it then writes out the full block
> JJ> again, both the already written and the new part.
>
> What does that mean for use of a flash SSD for the xlog dir?
>
> Does the block writing mesh up well with the usage pattern a flash
> SSD needs to maximize lifespan?

I think that SSD have a block size below which writing only part of
the block has the same effect as writing the whole thing. And those
block sizes are larger than 8K.
So PG always writing 8K at a time is unlikely to make a difference
than if it wrote a smaller amount.

Cheers,

Jeff