FAS2552 : using raid4 instead of raid dp

[IMPI]

Hi there,

we have a FAS 2552 with 20x900GB HDD and 4x400GB SSD.

We tried with original os : 8.3 and we also updated it to 9.1, same thing.

We'd like to save some disk space by using raid4 instead of raid dp (not a big danger, there is still a spare disk !)

When we try to do a "storage aggregate modify -raidtype raid4 ... "  on root aggr we are told this is impossible.

When adding a new aggr we can not choose raid 4 under GUI and CLI still saying "not possible".

Is there a way to have raid 4 ?

Would it be better to remain in raid dp and have no spare disks ? But is it possible... !?

Thanks for your help

[asulliva]

Hello @IMPI,

You can seamlessly update from RAID-4 to RAID-DP, but not the other way around.  While the reprotection time will be relatively minimal (it changes depending on the size and speed of the disk as well as how busy the controller is) with a spare disk being available, you are also increasing your risk of "silent bit rot" (exhaustive details) with only a single parity check being available on the aggregate(s).

It is possible to have the system operate without spare disks, however you will receive alarms about being in that state.  Have you reached out to your NetApp account team to discuss your capacity concerns?

If you're not already, make sure you are using Advanced Drive Partitioning (ADP), which uses shared root and data disks increasing system efficiency.

Hope that helps,

Andrew

[IMPI]

Andrew, many thanks for this quick and detailed reply.

In fact i should have enought space but it makes me crazy to loose 6 disks, this is really too much security for SMB enterprise. I prefer having more space.

You 're totally right about silent bit rot, i have totally forgot such concern, then raid dp without spare seems a better choice.

On dot 7-mode there were a "raid timeout option" that shutdowns the controller after an amount of time (24 hours by default)... i had it at a customer 🙂

I do not want such a thing on the system, for sure.

We are using ADP to avoid "wasting" system disks.

Regards

edit : i just read an excelent paper on the new ADP that cdot 9 uses : root-data-data (instead of 'old' ADP root-data). It seems good as i can use active/active controler AND have 2 parity and one spare for all the 20 disks !

I do not know if this can be used on a fas 2552 (it is writen 'not on entry fas') and if mixed hdd/ssd could be used. 

[andris]

Unfortunately, the root-data-data partitioning scheme is only available on 9.0+ AFF systems.

Are the 4 SSD's being used for FlashPool?  You can comfortably use RAID4 for them.

I would stick with RAID-DP for the HDD's - reliability levels of HDD's really demand it.

[D_BEREZENKO]

If for some reason you are not able or do not want to use ADP, you can convert existing DP aggregate to RAID4 (as long as upgraded aggregate fit into RAID4).

For example, you can convert DP of 3 disks (1 data + 2 parity) or 5 DP (3data+ 2parity) to RAID4, but you can not convert 28 drive DP to RAID4.

For new data aggregates you can use force-small-aggregate key:

cluster1::*> aggr create -aggregate data1 -diskcount 2 -raidtype raid4 -force-small-aggregate true

Note RAID4 is not supported on top of ADP.