Re: ZFS melting under postgres...

On Sun, Dec 16, 2007 at 08:40:18PM +1100, Darren Reed wrote:
> 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.

You missed the point:
The filesystem doesn't overwrite written data, but the media does
internaly to manage itself.
You can loose data which hasn't beeen writen at all, since there is a
large dependency chain with flash media.

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

Yes there is, because that's the way flash media works.
You write block x and if something goes wrong y is unreadable as well.
And those dependency areas are very hughe.

-- 
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