Re: Mis-Design of Btrfs?

[Ric Wheeler]

On 07/15/2011 02:20 PM, Chris Mason wrote:
> Excerpts from Ric Wheeler's message of 2011-07-15 08:58:04 -0400:
> > On 07/15/2011 12:34 PM, Chris Mason wrote:
> [ triggering IO retries on failed crc or other checks ]
>
> > > But, maybe the whole btrfs model is backwards for a generic layer.
> > > Instead of sending down ios and testing when they come back, we could
> > > just set a verification function (or stack of them?).
> > >
> > > For metadata, btrfs compares the crc and a few other fields of the
> > > metadata block, so we can easily add a compare function pointer and a
> > > void * to pass in.
> > >
> > > The problem is the crc can take a lot of CPU, so btrfs kicks it off to
> > > threading pools so saturate all the cpus on the box.  But there's no
> > > reason we can't make that available lower down.
> > >
> > > If we pushed the verification down, the retries could bubble up the
> > > stack instead of the other way around.
> > >
> > > -chris
> > I do like the idea of having the ability to do the verification and retries down
> > the stack where you actually have the most context to figure out what is possible...
> >
> > Why would you need to bubble back up anything other than an error when all
> > retries have failed?
> By bubble up I mean that if you have multiple layers capable of doing
> retries, the lowest levels would retry first.  Basically by the time we
> get an -EIO_ALREADY_RETRIED we know there's nothing that lower level can
> do to help.
>
> -chris

Absolutely sounds like the most sane way to go to me, thanks!

Ric

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