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How many disks can fail in 3PAR 7000 series?

 

Re: How many disks can fail in 3PAR 7000 series?

When writing, RAID-6 has a higher backend IO requirement (6 2/3 IOs) than RAID-5 (4 IOs).

 

RAID-5 has a 4 IO back end penalty.  As a write comes in the old parity and old data must be read (two read IOs) then parity calculated and new parity and new data written out (two write IOs).  So each random write to R5 has 4 back end IOs (2 read + 2 write).

 

RAID-6 calculation uses the XOR engine in the ASIC but must calculate two distinct parities (R5 only needs one parity) and need to compute parities over more data since RAID sets are larger.
Most writes require only two parities.  However, a fraction (1/3 for step size 8), of the data blocks are used to compute 3 parities, so updating those blocks requires reading/updating 3 parities, hence the odd number 6 2/3 back end IOs for RAID-6.

 

Nevertheless, when using the large nearline (NL) drives, HP strongly recommends using RAID-6 and avoid RAID-5 due to the long rebuild time when and if a NL drive fails.

 

 

Note: While I work for HP, all of my comments (whether noted or not), are my own and are not any official representation of the company.
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Note: While I am an HPE Employee, all of my comments (whether noted or not), are my own and are not any official representation of the company

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Prakash Singh_1
Esteemed Contributor

Re: How many disks can fail in 3PAR 7000 series?

Hi,

 

For HP 3PAR storage system querries you can also visit the HP Guided troubleshooting tree.

 

Below is the link for 3PAR HPGT:

 

http://h20180.www2.hp.com/apps/Nav?h_pagetype=s-905&h_lang=en&h_cc=us&h_product=304617&h_audience=smb&h_client=s-h-e010-1&h_audiencerestrict=true&lang=en&cc=us

 

 

HP Guided Troubleshooting Solving issues

Regards,

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huymeng
Senior Member

Re: How many disks can fail in 3PAR 7000 series?

Hi HP Pro

 

if raid 5 (3+1) CPG with 16 disks in same drive enclosure still can fail 1 disk

and if I choose raid 5 (7+1) CPG with 16 disks in same enclosure still can fail 1 disk

my better choice is raid 5 (7+1) because it can be usable capacity more than raid 5 (3+1)

 

I'm still not fully understand about How many disks can fail in 3PAR 7000 series?

 

appreciated for clarify

Dennis Handly
Acclaimed Contributor

Re: How many disks can fail in 3PAR 7000 series?

>If raid 5 (3+1) CPG can fail 1 disk

>if raid 5 (7+1) CPG can fail 1 disk

 

Yes.

 

>my better choice is raid 5 (7+1) because it can be usable capacity more than raid 5 (3+1)

 

Yes.  But if one disk fails, you now have to read 7 disks to recompute the data.

 

>I'm still not fully understand about How many disks can fail in 3PAR 7000 series?

 

Nothing has changed.  For RAID 1 and 5, just one disk, at the same time.  For RAID 6, just two, at the same time.

If there is a delay, the data can be copied to spare chunklets.

 

What are you trying to do/learn with this info?