Re: Re: TODO list

"Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
>> If the reason that a block CRC isn't on the TODO list is that Vadim
>> objects, maybe we should hear some reasons why he objects?  Maybe 
>> the objections could be dealt with, and everyone satisfied.

> Unordered disk writes are covered by backing up modified blocks
> in log. It allows not only catch such writes, as would CRC do,
> but *avoid* them.

> So, for what CRC could be used? To catch disk damages?
> Disk has its own CRC for this.

Oh, I see.  For anyone else who has trouble reading between the lines:

Blocks that have recently been written, but failed to make it down to
the disk platter intact, should be restorable from the WAL log.  So we
do not need a block-level CRC to guard against partial writes.

A block-level CRC might be useful to guard against long-term data
lossage, but Vadim thinks that the disk's own CRCs ought to be
sufficient for that (and I can't say I disagree).

So the only real benefit of a block-level CRC would be to guard against
bits dropped in transit from the disk surface to someplace else, ie,
during read or during a "cp -r" type copy of the database to another
location.  That's not a totally negligible risk, but is it worth the
overhead of updating and checking block CRCs?  Seems dubious at best.
        regards, tom lane