There are many reports on the Internet about cloud outage that caused massive losses to both providers and companies hosting their applications there (see here for the 10 worst according to Infoworld). One of the key concepts that TClouds is exploring is the use of multiple commodity clouds to form a cloud-of-clouds able to tolerate a large spectrum of problems. This concept was recently demonstrated by some TClouds researchers with the DepSky storage system, whose a paper appeared in the ACM EuroSys’11 conference.

Alysson Bessani, Miguel Correia, Bruno Quaresma, Fernando André and Paulo Sousa. DepSky: Dependable and Secure Storage in a Cloud-of-Clouds. In 6th ACM SIGOPS/EuroSys European Systems Conference (EuroSys'11), pp 31-45. April 2011.

In the paper, the researchers show a new data replication protocol that can be used to explore the fault independence of different storage cloud providers to implement a shared data storage that support updatable arbitrarily-sized object storage. These new protocols make use of techniques like secret sharing and erasure codes to provide confidentiality and storage-efficiency for the stored data.

The DepSky system was implemented on four commercial storage clouds (Amazon S3, Rackspace Files, Windows Azure Blob Service, Nirvanix CDN) forming a cloud-of-clouds trusted storage system. Experiments were performed with clients spread around the planet to assess the merits and drawbacks of the cloud-of-clouds approach for storage. The results show that the use of erasure codes make DepSky storage monetary costs from 23% to 50% more than single cloud storage (on average). In terms of performance, the DepSky read protocol presents better latency than any single cloud (data is fetched from the faster cloud at the moment) while the write protocol latency is similar to the worst cloud write latency (data needs to be written not on a single cloud, but on a quorum of them). Another interesting result show in the paper is the use of a cloud-of-clouds (with the 4 mentioned providers) make the perceived availability of the service is almost 100%, a fact that were not observed for any individual cloud storage provider.