EMC – My dog ate my SPS but I still need to use write cache

Someone contacted me via my About page a little while ago to ask how to use write cache on their CX without having an SPS. It’s worth noting that doing this outside of a lab environment is a really bad idea, as the point of the SPS is that you’ll have sufficient time to flush cache to disk prior to losing power to the array. But anyway, in case you missed it, here’s what to do.

Use EMCRemote to access the console of the SP. If you’re using a CX4, you can get some instructions on how to do that here. Fire up a command prompt, then run flarecons d f a (to access SP A). Then run setcache -nosps (to override the SPS state). You should then be able to enable write cache.

CLARiiON cabling …

With the introduction of the CX3 range of EMC CLARiiON arrays a few years ago, EMC introduced a new style of connector for the SPS and service port cables. They call it micro-DB9. An example of the connector is here:

The cable is plugged into the SPS monitoring port on the CX3 Storage Processor using this cable:

This all seems fairly straight forward. Unfortunately, if you were new to CLARiiON implementations and used the following diagram to guide you, you’d run into trouble. Because the service ports also use micro-DB9, rather than the RS-232 traditionally seen on earlier CLARiiON arrays.

If you follow this, you’ll plug the SPS sensor cable into the CX3 service port. Which won’t do any damage, but it won’t do any good either. The CLARiiON won’t know that the Standby Power Supply is connected, so it won’t let you enable the write-cache on the array. Things. Will. Be. Slow. You’ll probably also notice that the SPS have a little “E” next to them in Navisphere. This isn’t a trick. It means the array can’t see the SPS. It also probably wouldn’t hurt to plug the power in to the DAEs consistently, as you really want both PSUs on the first DAE holding the FLARE disks to be plugged in to the SPS. EMC puts it thusly:

It seems like simple stuff, but when the array is sitting in a co-lo and you only have limited “face time” with the array to do code upgrades, you don’t want to be wasting time with cabling issues.