Wednesday, May 8, 2013

Increasing FAST Cache on EMC VNX5300

Adding FAST cache onto a VNX SAN is one of cheapest, easiest and quickest ways to increase the performance of the storage system as a whole. The steps to upgrade the cache is painless and simple and are documented below.

Prior to updating the FAST cache, please take into consideration the following:

  • the maximum FAST cache capacity varies depending on each model. The chart below indicates the maximum capacity and the permissible flash drive counts.  

System Model
FAST Cache
drive count
Permissible Flash Drive Count
for FAST Cache

100 GB

500 GB
2*, 4, 6, 8 or 10 (100 GB EFD)
or 2* or 4 (200 GB EFD)

1000 GB
All VNX5300 configurations, plus:
12, 14, 16, 18 or 20 (100 GB EFD)
6, 8 or 10 (200 GB EFD)

1500 GB
All VNX5500 configurations, plus:
22, 24, 26, 28 or 30 (100 GB EFD)
12 or 14 (200 GB EFD)

2100 GB
All VNX5700 configurations, plus:
32, 34, 36, 38, 40 or 42 (100 GB EFD)
16, 18 or 20 (200 GB EFD)

* Not recommended

Please note that FAST cache can only be configured in RAID1

  • all drives in the FAST cache should be of the same capacity
  • try to spread the FAST cache disks across different buses

With those considerations in mind, we upgraded our VNX5300 from 200 GB to 400 GB by adding an additional two 200GB EFDs. In order to upgrade the cache, you must actually destroy the current cache and then recreate it. 

First, log in to Unisphere and open the System Properties. Under the "FAST Cache" tab, hit destroy.

The VNX will then flush all the cached data to disk. You will see the following warning:

Click yes and proceed. It will take a bit of time for all the data to be flushed out. The Fast Cache tab will now show the status of the destroy operation. You will notice the "Percentage Dirty Pages (%)" counters begin to decrease.

It should look like this when it is done.

Next, hit create and then manually select your EFDs to be used for the FAST cache.

The cache initializing process will now begin. As with the destroy operation, the initialization process status will be shown in the FAST cache tab.

After a while, the FAST cache will be initialized. You may want to check your RAID groups to ensure that FAST cache is still enabled for the LUNs residing in RAID groups.


  1. Thanks for the guide! About how long did it take to destroy the existing cache and how long did it take to rebuild once you added the extra drives? I'd like to do this after hours in a maintenance window but I'd like to make sure i have enough time.