ONTAP Hardware

Can I change raid type from "raid4" to "raid-dp"?

kurtgandy21

Can I change raid type from "raid4" to "raid-dp" on FAS3020 runing Ontap 7.3.3. without data disruption using the cli ?

11 REPLIES 11

spence

You can if you have spare disks of the same type (AT\FC\SAS) with equal or greater capacity.

Chris Spence

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kurtgandy21

Nah don't spare disks with equal or greater capacity. Well I did a backup and I want remove/redo aggr0 and set to Raid DP what's tha fastest way?

shane_bradley

Assuming you have a spare disk to become the "DP" drive then yes this can be added and removed on the fly.

If you dont have the disk then unfortunately you have to migrate off destroy the aggr then recreate.

Hi,

following up on this thread, I have a FAS2020, which has a spare disk already, and is in RAID-4. When I try to change to "dp" it warns me:

=== DETAILS ===
WARNING! Continuing with vol options aggr0 raidtype raid_dp will result in a low spares disk condition for one or more RAID groups.
=== CORRECTIVE ACTION ===
No corrective action found

Also, my aggregate has 5 disks + 1 spare. Shouldn't I be able to change to "dp" ?

Thanks,

Nuno

chriskranz

Remember you'll still need a hot spare available after you've converted the aggregate to RAID-DP. If you only have 1 spare today, you won't be able to do this without buying more disks I'm afraid. Well, you can, but you'll have no hot spares and I'd rather have RAID-4 than no hot spares. A disk rebuild is quicker and more efficient in most scenarios than doing parity calculations on each read. It's a fine line, but I'd stick with RAID-4 unless you can buy more disks.

aborzenkov

Personally I’d rather have RAID_DP without spares ☺

chriskranz

Why?

RAID-DP over RAID-4 has no read or write penalty, arguably RAID-4 only has one parity calculation, but the write cache removes that as a real concern.

Most disk failures are proactively failed, and a disk-to-disk copy has little or no impact on the CPU. So my RAID-4 system will have very little performance hit on a disk failure. I can still suffer 2 failures, although not at the same time.

In a RAID-DP system I have 2 parity disks but no hot spare, so any failure, proactive or otherwise, will cause a parity calculation, and this will have to be done for EVERY read until the failed disk is replaced. When the failed disk is replaced I then have to do an entire parity recalculation as I have no disk to copy valid data from.

Keep in mind that my thinking above is only valid for a small 2000 series with 12 disks where the RAID group sizes are also small. I'd vote RAID-DP every single time on a larger system without fail.

aborzenkov

Because I have seen reconstruction failures caused by latent sector failures, and my priority is data availability above data access speed ☺ Such failures are probably less likely on NetApp due to the way data is placed on disks … but old habits die hard ☺

Proactive disk sparing requires 2 spares so it is out of question here anyway.

chriskranz

When I say proactive disk sparing, I mean when a disk is failed due to the quantity of bit level or software errors, rather than catastrophic disk failure. This doesn't require 2 disks, as far as I remember, 2 disks are only required for the maintenance garage which will run low level checks and reformat disks and bring them back into production based on software errors. But I could be completely wrong there

NUNOPEDROSILVA

Hi everyone,

Thanks on the feedback given. It's now much more clearer.

Nuno

aborzenkov

You should as long as you still have enough spares left after this. Converting to RAID_DP consumes one spare per raid group, and you most probably have just one anyway.

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