Power Issues

Question

I am currently running an AX-150 with two Fibre channel switches (Brocade SilkWorm200s), and there is an upcoming event that may have some problems I'd like to clear up.   Currently, SPB2's power supply is connected through the rack's power strip into an APC 1500 SmartUPS.  The SPB1 power supply is connected directly to a SmartUPS750 that is dedicated to it alone.   The 1500 SmartUPs that supports SPB for the EMC2 is due to be taken out of service briefly next week to replace the battery, and so there will be no power for a short period to it.   I have been looking through my AX-150s documentation and also the forum page but did not see what I'm after, which is a recommended process to handle the interrupt of power to the SPB.   I do have another UPS that I can switch SPB for the duration, but it will still end up being disconnected no matter what. In reading the documentation, it states that there will be a 'degraded status' until it regains power, but what is the best procedure to make sure there are no problems whilst moving the power supply's connection from one UPS to the other?

Dev Mgr · Answer

If only 1 powersupply is receiving power, the write cache disables itself, but the system will continue to function just fine.

If you wish to limit this performance-hit-window, pull the powercable from your APC1500 and move it to a different powersource temporarily (e.g. one of the PDUs in the rack). Then, when you've done the work you needed to, move the powercable back.

PittWolfBW · Answer

Thanks! I was looking for a specific example of what would happen when one of the SP's power fails, beyond the 'operates in a degraded state' from the documention. I am not concerned with the performance hit, though, as we've made arrangements for this to be done when the file server that the array services will not be in use.

One other question - currently, SPA is hooked into the UPS 750 that came with the array, while SPB is connected to a separate UPS. (SmartUPS1500). In the case of a power failure, the documentation describes that the UPS would automatically provide power to the SP it supports for it to finish writing any cached data to disk, and then powers the SP down into what it describes as a sleep mode.

What I want to know is, once the UPS is out of power, what conditions will occur with the array? Will the disks automatically shut themselves down and lock once no power at all is coming from the UPS? Since I'm using two separate UPS here (the SmartUPS 750 that came with the array, and a SmartUPS 1500 that hooks to SPB), is it the array that drives these actions, or the UPS - and as such, will both UPS perform in the same fashion?

Message Edited by PittWolfBW on 01-22-2008 09:30 AM

Dell-Jeff G · Answer

The AX150's UPS is designed to keep the array powered up only long enough to flush cache and then shut down.

PittWolfBW wrote:

What I want to know is, once the UPS is out of power, what conditions will occur with the array?

Since you have a UPS connected to SPB, if you loose power to both UPS's, the one connected to SPA will notify the array it has lost power and write cache will be disabled. If power is restored before the battery is drained, then write cache will be enabled. If the battery is drained, then the system will just shut off when the battery dies. When power is restored, the system will turn back on and enable write cache once the battery to the UPS connected to SPA is charged.

Will the disks automatically shut themselves down and lock once no power at all is coming from the UPS?

Since there is no power, the disks will spin down....

Since I'm using two separate UPS here (the SmartUPS 750 that came with the array, and a SmartUPS 1500 that hooks to SPB), is it the array that drives these actions, or the UPS - and as such, will both UPS perform in the same fashion?

Both UPSs are not acting in the same fashion. Since only SPA is communicating with the UPS connected to it. SPB is not communicating with the UPS it is connected to, it is just providing power to the SPB.

I have seen some customers connect SPA's UPS to a larger UPS, such as a SmartUPS 3000, that way in a power failure, the EMC SmartUPS 750 never looses power and disables write cache. If power is lost long enough that the 3000's battery is drained, then the 750 kicks in and disables write cache.

Some customers have the entire server room on a very large UPS and don't think they need the EMC SmartUPS 750 for the AX150, but that is not the case. In order to have write cache, it must be used. Keeping in mind, it's purpose is to keep the system up long enough to flush cache (2 minutes)

PittWolfBW · Answer

Thank you very much, that pretty much lays out everything I needed with one last question.

Only the SmartUPS 1500 that connects to SPB is going to be powered off for a brief period to install a NEtwork Management card, and I was concerned about all potential effects of this on the array. The SmartUPS 750 that SPA connects to is not going to be affected in any way by this. Since only the non-communicating UPS on SPB is powering down, will write caching still be disabled? SPA will again not be affected in any way.

Thanks again!

Message Edited by PittWolfBW on 01-22-2008 03:56 PM

PittWolfBW · Answer

No, definitely not. The Network management card is going in the larger UPS (1500) that services SPB as well as several other devices on the network.   Thanks very much for all the information!

Dev Mgr · Answer

Write cache disables if 1 powersupply loses power.

So if your APC1500 is powered down (for the adding of the management card), PSU-B loses power, and therefor you lose write cache.

Note: do NOT put a management card in the APC750 (that came with the AX150), as this will prevent the AX150 from being able to communicate (via serial) to the UPS and cause it to disable write cache.

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