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EMC Symmetrix DMX device type, COVD: Cache Only Virtual Device

On Twitter this morning we (@chrismevans@basraayman@davegraham) were discussing device type: COVD (Cache Only Virtual Device) on EMC Symmetrix DMX platform.

So here is some information on Cache Only Virtual Devices. I do not have a very clear picture on the overall operation of this device type, but from a high level it can be summed up as following based on it characteristics.

Starting with microcode 5670 on EMC Symmetrix DMX Systems, EMC introduced COVD (device types). We have seen instances of COVD on 5671, 5771 and 5772 microcodes, really unknown if they exist on EMC Symmetrix V-Max systems at this point.

Here are some highlights on COVD:

  • Even though COVD’s were introduced on the 5670 microcode, recommendation is to upgrade to 5671 on the R2 side of SRDF/A before implementing COVD’s.
  • Used with SRDF/A technology for caching data on R2 side.
  • Symconfigure will not allow (block) you to change SRDF/A group on R2 side for COVD devices. You will need a BIN File change for this process by the Customer Engineer.
  • COVD is a Virtual Device but does end up taking two device numbers within your list of Symmetrix device numbers (I believe 8192 device numbers are available on the early DMX’s).
  • If you are using COVD, your configured capacity might show more than your Raw Capacity in ECC and StorageScope.
  • COVD’s cannot be snapped using TImefinder
  • COVD’s can only be created and destroyed by BIN File (not through SYMCLI)
  • COVD is only found on R2 side of SRDF/A
  • Cache is used as part of creating the COVD
  • COVD’s are used in pairs, one is used for active SRDF/A cycle and 1 is used for inactive SRDF/A cycle
  • No Data is stored on COVD, used practically for caching
  • Primarily introduction of COVD was to reduce the write pending limits with SRDF/A

Haven’t really seen a lot of customers using COVD (device types). But sometimes during storage analysis of customer meta data reveals these device types since it is assigned a device number.