Dave Graham posted an interesting article on his blog yesterday, relating to a new product from Emulex. Called E3S or Emulex Enterprise Elastic Storage, the appliance (as it appears to be being positioned) allows block-level data to be migrated into the cloud for later access.
Now there are a few interesting points here. First, the discussion related to block-level data, that is data written to and from LUNs rather than files. Second, data is maintained consistently (“consistent copies” to quote Dave). Third, data is encapsulated by the E3S device and returned back to the host as required.
I’m a little confused as to what the offering actually is; is it a backup solution; is it a replication solution? Let’s think about this in more detail.
Anyone who knows how large enterprise arrays work will know data consistency and integrity is king. The array itself has no concept of the format of the file system written onto the LUNs and consequently storage arrays must maintain write sequence integrity in order to maintain file system consistency. This isn’t a problem with a single array and synchronous replication as the I/Os are written to cache in timestamp order and replicated to remote storage arrays in the same format. Asynchronous replication operates in a similar manner, except that LUNs are grouped together based on their consistency requirements — usually all the LUNs presented to a single host or application. If write order is not maintained or data fragments are lost, then the target copies can be rendered completely useless.
Data Integrity With Unreliable Transportation
LUN replication requires consistency and data integrity. Replication into the cloud occurs across an IP connection which doesn’t have the same performance guarantees and reliability as a dedicated point-to-point network like Fibre Channel. The problem therefore, is to move data reliably to the cloud and provide integrity checking, packet resends and manage an unpredictable performance profile.
IP-based replication technology which deals with the performance and reliability issues already exists in the market today. In fact, EMC have a product doing just this — RecoverPoint. They also have Open Replicator, which writes data to unlike devices across fibre channel networks. There’s also Axxana’s Phoenix appliance which can move data across mobile networks. So what exactly are Emulex offering that makes it unique compared to these technologies in the marketplace today? We can only wait to find out.
So what questions should be asked of this kind of replication technology? Here’s a few:
- How is data integrity maintained in replicating LUNs to “the cloud”?
- Does the E3S appliance cache data as it is forwarded to “the cloud”?
- What redundancy and integrity is built into the E3S appliance to ensure no data loss?
- What level of throughput can the E3S appliance maintain?
- How is multi-LUN replication integrity managed?
- How is multi-array replication integrity managed?
- Can data be accessed directly from “the cloud”?
There are a couple of other posts out there from Chris Mellor and Beth Pariseau. You can find them here: