Re: BTRFS for OLTP Databases

[Kai Krakow <hurikhan77@gmail.com>]

Am Tue, 7 Feb 2017 15:27:34 -0500
schrieb "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:

> >> I'm not sure about this one.  I would assume based on the fact that
> >> many other things don't work with nodatacow and that regular defrag
> >> doesn't work on files which are currently mapped as executable code
> >> that it does not, but I could be completely wrong about this too.  
> >
> > Technically, there's nothing that prevents autodefrag to work for
> > nodatacow files. The question is: is it really necessary? Standard
> > file systems also have no autodefrag, it's not an issue there
> > because they are essentially nodatacow. Simply defrag the database
> > file once and you're done. Transactional MySQL uses huge data
> > files, probably preallocated. It should simply work with
> > nodatacow.  
> The thing is, I don't have enough knowledge of how defrag is
> implemented in BTRFS to say for certain that ti doesn't use COW
> semantics somewhere (and I would actually expect it to do so, since
> that in theory makes many things _much_ easier to handle), and if it
> uses COW somewhere, then it by definition doesn't work on NOCOW files.

A dev would be needed on this. But from a non-dev point of view, the
defrag operation itself is CoW: Blocks are rewritten to another
location in contiguous order. Only metadata CoW should be needed for
this operation.

It should be nothing else than writing to a nodatacow snapshot... Just
that the snapshot is more or less implicit and temporary.

Hmm? *curious*

-- 
Regards,
Kai

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