[dm-devel] RAID4 with no striping mode request

[Heinz Mauelshagen]

Roger,

as any of the currently implemented 'parity' algorithms (block
xor/P-/Q-Syndrome) provided by DM/MD RAID
have to have at least two data blocks to calculate:  are you, apart from
the filesystem thoughts you bring up, thinking
about running those on e.g. pairs of disks of mentioned even numbered set
of 8?

Heinz

On Tue, Feb 14, 2023 at 11:28 PM Roger Heflin <rogerheflin at gmail.com> wrote:

> On Tue, Feb 14, 2023 at 3:27 PM Heinz Mauelshagen <heinzm at redhat.com>
> wrote:
> >
>
> >
> >
> > ...which is RAID1 plus a parity disk which seems superfluous as you
> achieve (N-1)
> > resilience against single device failures already without the later.
> >
> > What would you need such parity disk for?
> >
> > Heinz
> >
>
> I thought that at first too, but threw that idea out as it did not
> make much sense.
>
> What he appears to want is 8 linear non-striped data disks + a parity disk.
>
> Such that you can lose any one data disk and parity can rebuild that
> disk.  And if you lose several data diskis, then you have intact
> non-striped data for the remaining disks.
>
> It would almost seem that you would need to put a separate filesystem
> on each data disk/section (or have a filesystem that is redundant
> enough to survive) otherwise losing an entire data disk would leave
> the filesystem in a mess..
>
> So N filesystems + a parity disk for the data on the N separate
> filesystems.   And each write needs you to read the data from the disk
> you are writing to, and the parity and recalculate the new parity and
> write out the data and new parity.
>
> If the parity disk was an SSD it would be fast enough, but if parity
> was an SSD I would expect it to get used up/burned out from all of
> parity being re-written for each write on each disk unless you bought
> an expensive high-write ssd.
>
> The only advantage of the setup is that if you lose too many disks you
> still have some data.
>
> It is not clear to me that it would be any cheaper if parity needs to
> be a normal ssd's (since ssds are about 4x the price/gb and high-write
> ones are even more) than a classic bunch of mirrors, or even say a 4
> disks raid6 where you can lose any 2 and still have data.
>
>
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