Re: Checkpoint cost, looks like it is WAL/CRC

Zeugswetter Andreas DAZ SD wrote:
> 
> >> Are you sure about that? That would probably be the normal case, but 
> >> are you promised that the hardware will write all of the sectors of a
> 
> >> block in order?
> > 
> > I don't think you can possibly assume that.  If the block 
> > crosses a cylinder boundary then it's certainly an unsafe 
> > assumption, and even within a cylinder (no seek required) I'm 
> > pretty sure that disk drives have understood "write the next 
> > sector that passes under the heads"
> > for decades.
> 
> A lot of hardware exists, that guards against partial writes
> of single IO requests (a persistent write cache for a HP raid 
> controller for intel servers costs ~500$ extra).
> 
> But, the OS usually has 4k (some 8k) filesystem buffer size,
> and since we do not use direct io for datafiles, the OS might decide 
> to schedule two 4k writes differently for one 8k page.
> 
> If you do not build pg to match your fs buffer size you cannot
> guard against partial writes with hardware :-(
> 
> We could alleviate that problem with direct io for datafiles.

Now that is an interesting analysis.  I thought people who used
batter-backed drive cache wouldn't have partial page write problems, but
I now see it is certainly possible.

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