Re: Is there a performance advantage of adding a second parity set?

[John Jones <chianime@xxxxxxxxx>]

If you add 6 drives at once the parity should spread across four of them as
you presume.

I don't know that the performance difference would be measurable but I would
hazard a guess that 2 parity sets would be better than one.  With just one
set those 8 parity drives will be spending more and more time performing
parity writes for the additional drives; on average they will be busier
which could induce delays when the OS wants to read from them.  More parity
sets spreads the parity writes across more drives sort of like how the i
scatter-loads data to begin with.  The number of parity writes is unchanged
but the busy-ness of the drives that have parity duty is reduced.

Also, a second parity set increases your reliability as you could lose one
drive in each set and still be operational.

On modern RAID cards, what is the maximum number of drives per parity set
anyway?  I thought it topped out at 15 or 18.

On Mon, Feb 15, 2010 at 8:18 AM, <rob@xxxxxxxxx> wrote:

> We have a system with 17 drives.  We are going to add six 70GB drives.  If
> I add them two at a time they will add them to the existing parity set. If
> I add them all at once it will start a new parity set.  Starting a new
> parity set effectively ties up one of the 70GB drives to raid striping. We
> really don't need the space though - we just have 70GB drives laying
> around.  Without the new drives we are only at 50% used.  The existing
> parity set has the striping across 8 drives.  I take it the new set would
> have the striping across 4 drives?
>
> These six 4327's would go into a 5787.  Into P1-D1 through P1-D6.
> P1-D7 through P1-D12 are 4328's.  P2-D1 through P2-D5 and P2-D7 through
> P2-D11 are 4328's.  Leaving P2-D6 and P2-D12 open.
>
> Rob Berendt
> --
> Group Dekko Services, LLC
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>
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