Re: [PATCH 009 of 11] md: Support stripe/offset mode in raid10

[Al Boldi <a1426z@gawab.com>]

Neil Brown wrote:
> On Tuesday May 2, a1426z@gawab.com wrote:
> > NeilBrown wrote:
> > > The "industry standard" DDF format allows for a stripe/offset layout
> > > where data is duplicated on different stripes. e.g.
> > >
> > >   A  B  C  D
> > >   D  A  B  C
> > >   E  F  G  H
> > >   H  E  F  G
> > >
> > > (columns are drives, rows are stripes, LETTERS are chunks of data).
> >
> > Presumably, this is the case for --layout=f2 ?
>
> Almost.  mdadm doesn't support this layout yet.
> 'f2' is a similar layout, but the offset stripes are a lot further
> down the drives.
> It will possibly be called 'o2' or 'offset2'.
>
> > If so, would --layout=f4 result in a 4-mirror/striped array?
>
> o4 on a 4 drive array would be
>
>    A  B  C  D
>    D  A  B  C
>    C  D  A  B
>    B  C  D  A
>    E  F  G  H
>    ....

Yes, so would this give us 4 physically duplicate mirrors?
If not, would it be possible to add a far-offset mode to yield such a layout?

> > Also, would it be possible to have a staged write-back mechanism across
> > multiple stripes?
>
> What exactly would that mean?

Write the first stripe, then write subsequent duplicate stripes based on idle 
with a max delay for each delayed stripe.

> And what would be the advantage?

Faster burst writes, probably.

Thanks!

--
Al