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VNX2 and drive placement bus enclosure


I am not very clear on how the bus and enclosures scheme is seen from a design point of view and considering drive placement on a VNX5600.

I have seen in the latest VNX2 best practice document the below for RG and drive placement.

When selecting the drives to use in a Classic RAID group, drives can be selected

either from a DAE or DAEs on the same bus (horizontal positioning), or from multiple

DAEs on different buses (vertical positioning). There are now almost no performance

or availability advantages to using vertical positioning. Therefore:

  • Use the default horizontal positioning method of drive selection when creating RAID groups.
  • If a single DAE does not have enough available drives to create the RAID
  • group, selecting drives from a second DAE is acceptable.
  • The SAS buses are fast enough to support drives distributed across  enclosures belonging to the same RAID group.

However for Flash Drive placement it says:

Physical placement of drivesWhen initially placing drives in the array:

  • Spread flash drives across all available buses, and when possible place them in the lowest-numbered enclosures
  • There are no restrictions around using or spanning across Bus 0 Enclosure 0

I know the DPE contain a 25 x 2.5" enclosure.  Is this considered a bus 0 and enclosure 0 and the enclosure has 25 slots?

And is an additional DAE e.g 15 x 3.5" considered an enclosure on a separate bus?

Can someone please help me to understand the above, respective to a VNX 5600 and drive placement for both a RG and Storage Pool approach.  Meaning the advantages or disadvantages or placing drives sequentially across enclosures opposed to across buses and enclosures.

I ask also in the event that if you were to lose 1 pwr supply on a DAE and while waiting on another pwr supply to be replaced, you lose the other supply.  You may loose an entire RG or arrays because of this if drives are placed sequential on enclosures and opposed to vertical on different enclosures?

Much appreciated

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4 Beryllium

Re: VNX2 and drive placement bus enclosure

Simply put, spinning drives can placed across any DAE.  You can create private RAID groups for storage pools from spinning drives in any DAE.  You can also span RAID groups across DAEs (vertically) as-needed.

For Flash drives, however, the recommendation is to spread drives across buses and enclosures, as many flash drives can easily overwhelm a backend bus.  The physically cabling of your VNX5600 determines the addressing of its enclosures.  You can see this in Unisphere in the Hardware submenu.  Your DPE is indeed Bus 0 Enclosure 0, but your next DAE may or may not be on the same bus.  You'll need to check the configuration and see.

If you have a RAID group that spans an enclosure that looses power, you will lose data access to the entire pool.   This is also true of a storage pool that uses multiple enclosures - if you lose access to any of the private RAID groups that make up the pool, you will be unable to access LUNs within the pool, because the data is distributed across all the private RAID groups that make up the pool.  You will not regain access until the DAE is powered up and the VNX has re-verified the data blocks.  For this reason alone, I would not hesitate to fix a failed power supply as soon as it's faulted.

5 Rhenium

Re: VNX2 and drive placement bus enclosure

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