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Storing files in a scaled-out fashion is a pain in the NAS (part 2) - The BLOB service approach to file management

3/2/2011 10:22:08 PM
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2. The BLOB service approach to file management

As we discovered earlier, the BLOB storage service is the Windows Azure solution to providing file storage. Let’s take a look at how Azure implements this service.

An API-Based Service

Rather than building a native network-share-based solution, Microsoft has provided a set of REST-based APIs that allow you to interact with all the storage services over the HTTP stack, using a standard HTTP request. As mentioned earlier, not only can you use these APIs inside the data center, but you can also use them outside the data center.


Although you can upload and download files outside the data center, you’ll be subject to internet speed; it might take you a few hours to upload or download gigabytes of data. Within the data center, you can copy gigabytes of data between BLOB storage and a worker or web role in seconds. This massive speed difference is the result of the co-location of the storage service and the roles.


Using HTTP as the underlying transport layer means that Windows Azure can leverage the web role infrastructure inside Windows Azure to host the storage services. By using the web role infrastructure to host the Windows Azure storage service (with tens of thousands of instances), you can be confident that your application will be able scale to that level. Figure 4 shows the abstraction of web instances for the BLOB storage service.

Figure 4. Scaling of BLOB storage services in Windows Azure

Because BLOB storage is built on the web role infrastructure, web roles can also harness the advantages of utility computing. As the demand for the storage services increases, Microsoft can ramp up the number of instances just like it can for any other web role. You don’t need to worry about the scalability of any of the storage services (unless Microsoft runs out of pennies).

Disk Storage

Just as there are thousands of racks of machines used to host the web and worker roles, there are just as many disk arrays storing your data! Microsoft can grow the storage required in the data center by adding more disks as and when required. This level of enterprise-class storage means that you never need to worry about capacity or scale. Think of the BLOB service as a giant virtual hard disk that will always scale up to meet your demands and never run out of space.

Data Consistency with Replication

Like the DFS solution, Windows Azure BLOB storage is also a replicated solution (to be honest, you have to be to achieve such massive scale). Although the BLOB service is quite similar to the Amazon Simple Storage System (Amazon S3), replication is one of the areas in which it differs.

With Amazon S3, there’s no consistency of data throughout the data center. If you upload a file to Amazon S3 and then request that same file, it’s likely that a different server will process that request. As a result of network latency, the file probably won’t be available to the new server because the data won’t have been replicated from the original server yet. Amazon S3 suffers from the same issues seen with DFS.

This issue of replication latency can never occur in Windows Azure storage services. Windows Azure guarantees a consistent view of your data across all instances that might serve your requests. Internally, inside the Windows Azure storage services, data is replicated throughout the data center as soon as it’s written to your storage account. Every piece of data must be replicated at least three times as part of the commit process.

As your data is being replicated across the various disks in Windows Azure, the FC keeps track of which instances can access the latest version of your data. The load balancer will route requests only to an instance that can access the latest version, ensuring that stale data is never served.

Even if a disk failure occurs immediately after the upload, there won’t be any data loss; other disks are guaranteed to receive a copy of that data.

So far we’ve talked about how BLOB storage solves the problems of scalability and fault tolerance, but we haven’t talked about performance. Surely performance is going to suffer; it’s effectively a REST-based web service, after all.


Sure, the performance of BLOB storage in comparison to SANs or DASs isn’t all that great. Ultimately that tradeoff between performance, fault tolerance, and scalability means that performance is lost. However, within the data center, it’s generally good enough performance. Because the service is ultimately a load balanced web server, you can expect 50 milliseconds to 100 milliseconds of latency between your role and the storage service. Although the latency is poor, the network connection is fast, so you can expect good enough performance. Sure, you wouldn’t allow an application that needs to write to disk very quickly (for example, SQL Server) to write directly to BLOB storage, but not all applications need that kind of speed.

If you do need that level of speed, you can always cache files locally on your role using local storage. This technique will usually give you more acceptable performance for your application. In fact, this is exactly what the Azure Drive (originally called X-Drive) feature uses to ensure performance.

What’s Azure Drive?

Although the REST API is flexible and provides great scale, it’s no substitute for a good old filesystem. To make life a little easier for those bits of code that are used to talk to directories and files rather than to a web service, Microsoft has provided a new feature called Azure Drive. Azure Drive allows you to mount BLOB storage as a New Technology File System (NTFS) drive, which lets you access BLOB storage just like any other drive. Because this feature is implemented using a special OS driver that was developed specifically for Windows Azure, this feature is only available to your roles; it’s not available outside the data center.

As cool as Azure Drive is, it allows only one instance of a role to read and write to the Azure Drive. Multiple role instances can mount the same Azure Drive, but only in a read-only mode, and only against a snapshot of the drive itself.

Now that we’ve looked at how BLOB storage handles the issues that arise in traditional on-premises solutions, it’s worth looking at BLOB storage from a data management perspective.


One of the most compelling arguments for using the Windows Azure storage services is that IT professional management skills aren’t required. In traditional systems, a large investment in IT management skills is usually needed to support storage. Management of the storage arrays usually requires expensive specialists who are capable of supporting the data, such as SAN experts, network specialists, technicians, administrators, and DBAs.

To plan such a system, these experts need to be able to design and implement the infrastructure, taking disk management, fault tolerance, networking, lights-out operation, and data distribution into consideration. The day-to-day running of the system includes hardware replacement, managing backups, optimizing infrastructure, health monitoring, and data cleansing, among other endless tasks.

With Windows Azure, you can let Microsoft manage the storage systems and concentrate on using the system via familiar developer APIs. You can focus on your core skill set, which is building software.

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