ZFS melting under postgres...

[Darren Reed]

Bernd Walter wrote:
...
> One problem is with the data blocks beeing that big, when writing
> 512 Byte you effectifly do a read-modify-write of a larger physical
> block.
> This can be handled quite well with larger FS block.
> The much bigger problem is with power loss when writing such a
> maintenence block.
> You loose a very large area of logical blocks when this fails,
> since a 4k maintenence block contains the allocation for several hundert
> kB of logical data blocks.
> In other words - you possibly loose data blocks that were not written
> a long time and the database wouldn't expect a problem with that data.
> Even for ZIL it is very questionable if you loose a large data area,
> since the purpose is to have the data that was already sinced readable
> after a power loss.
...

ZFS doesn't suffer from this problem because the design
is to always write a new section of data rather than
over write "current" data.

So if you lose power in the middle of a write to a data
block, there is no damage to the old data.

Darren