>> 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.

Andreas

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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.

--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073

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