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Storing files in a scaled-out fashion is a pain in the NAS (part 1) - Traditional approaches to BLOB management

3/2/2011 10:19:51 PM
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Unless you have plenty of cash, you’re going to experience some pain if you try to share files across machines. No matter what hat one of us puts on (author, presenter, architect, developer, or computer scientist), we’re embarrassed by the following statement: sharing files across machines is incredibly hard. It is; it shouldn’t be, but it is. Decoding the genome and making robots climb stairs, that should be hard, but sharing files shouldn’t be.

We wanted to provide a service where users could upload podcasts that would be converted from MP3 to WMA. To support the predicted demand, we decided to load balance the website across two servers. Because users can upload or download a podcast from any server, a shared storage solution is required.

Figure 1 shows a logical representation of two load balanced web servers accessing a podcast from a shared storage mechanism.

Figure 1. Two load balanced servers that require access to a common storage area to serve up MP3 files


To be honest, you don’t need to be the greatest architect in the world to draw the solution shown in figure 1. It’s pretty logical, common sense stuff. Two web servers access a common storage area.

Now you’re thinking, “Why did they just say it’s common sense, when before they said it was hard? Get me another book that says it’s easy.” Well, before you start reaching for Mavis Beacon Teaches Windows Azure, check out the following questions. As you think about the possible answers, you might begin to see why this is a little harder than it seems to be at first.

  • Do you have enough space to store all the files you need?

  • How do you add more storage capacity?

  • If a disk crashes, where does your data go?

  • Is the storage block load balanced?

  • What if you lose your connection to the block? Is it redundant?

  • At what point do you max out your disk, in terms of reading and writing?

  • How do you evenly distribute load across all disks?

The good news is that pretty much all of these problems have been answered and solved already. You can even implement these solutions in your traditional noncloud environments today (well, the lead time is probably longer than a day). The bad news is that the cheap, simple solutions are typically not scalable or fault tolerant. The solutions that are scalable and durable are usually expensive. In the Windows Azure BLOB storage service, all that changes.

Before we look at how easy it is to store and access files (in a scalable, durable fashion) across multiple servers in Windows Azure, let’s look at some of the options outside Windows Azure.

1. Traditional approaches to BLOB management

Over the next few sections we’ll look at how you might provide a file storage facility in traditional ASP.NET web server farms, using our podcasting example. We’ll specifically look at using the following storage options:

  • SQL Server

  • Network share

  • Distributed File System (DFS)

  • Network-attached storage (NAS)

  • Direct-attached storage (DAS)

  • Storage area network (SAN)

Let’s start with one with the typical developer solutions to the problem: the database.

SQL Server

Because web servers typically have access to a shared SQL Server database, you could store your podcasts in a table. Although this is a common approach used in many solutions, it’s probably not the best use of your expensive database server. It’s like racing a truck in a Grand Prix; there are cheaper, simpler, higher performing, and more appropriate solutions for storing files.

Unless you’re using a high-availability technology (such as clustering, mirroring, or replication), your database server is likely to be a single point of failure in the system. In figure 1, SQL Server would be represented by the Storage block (accessed over a typical network connection).

Network Share

Another common approach to providing a shared filesystem across web servers is to use a shared network drive that can be accessed by all instances of the website. This low-cost solution is more lightweight than a database, but it still introduces a single point of failure. This cheapo solution offers no redundancy and provides no ability to scale out. In figure 1, an application server with a network share would also be represented by the storage block.

Now that we’ve looked at some of the lower-end solutions, let’s take a look at some of the typical high-scale solutions that are used, starting with Distributed File Systems.

Distributed File System (DFS)

Windows Server 2003/2008 provides a technology known as DFS that allows you to create a peer-to-peer (P2P) filesystem on your network. UNIX/Linux environments have similar tools. If you use DFS to store podcasts, when a new podcast is uploaded, a copy of the file is replicated to all other participating servers. Although this approach requires no new hardware, it’s complicated to manage and adds extra performance overhead to all servers involved.

Figure 2 shows a DFS solution with a P2P network between two web servers.

Figure 2. Two web servers reading and writing files to a local shared disk. Files are replicated between each server.


Whenever a file is uploaded to a web server, it’s automatically replicated to all other servers in the farms. Using replication ensures that there are no single points of failure in this solution and that the data is held on multiple machines. In figure 2, Podcast01.mp3 is uploaded to web server 1 and then replicated to web server 2; when Podcast02.mp3 is uploaded to web server 2, it’s then replicated to web server 1.

In figure 3, the web servers don’t hold the files locally, but use a replicated file store held in application servers. In this figure, Podcast01.mp3 was uploaded to app server 1 via web server 1. The file was replicated to app server 2, and then served up to the client from app server 2 via web server 1.

Figure 3. Two web servers reading and writing files from a set of replicated file servers


With file replication, any time a file is uploaded to a server there’s a small delay between the file being uploaded and it being replicated across all servers. It’s therefore possible that the web user could be load balanced onto a server where the file isn’t available (because it hasn’t been replicated across to that server yet). Although this issue can be alleviated by using sticky sessions, sticky sessions won’t help if the original server keels over. Also, using sticky sessions means that incoming requests won’t be evenly distributed across all web servers.

Now that we’ve looked at some of the hook-some-machines-together solutions, we’ll look at some of the dedicated disk array–type solutions that are typically used in the market.

Sticky sessions

A sticky session occurs when a load balancer forwards all incoming requests from the same client to the same server for the period of the session.


Network-Attached Storage (NAS)

A network-attached storage device is a disk array that you can plug into your network and that can be accessed via a network share. NAS devices are responsible for managing the device hardware, the filesystem, and serving files, and can provide varying levels of redundancy, depending on the device and the number of disks in the array.

Although NAS devices reduce load from client operating systems by taking responsibility for file management, they can’t scale beyond their own hardware. NAS devices can range from being pretty cheap to very expensive, depending on the levels of scalability, performance, and redundancy that you require from the device. In figure 1, the NAS device would be represented by the storage block (connected via the Ethernet).

NAS devices are used to provide capabilities similar to those of a file server, rather than being used as a disk management system in a high-performance application solution.

Direct-Attached Storage (DAS)

A direct-attached storage device is a disk array that you can plug directly into the back of your server and that can be accessed natively by the server. DAS devices are responsible for managing the device hardware and can provide varying levels of redundancy, depending on the device and the number of disks in the array.

Because DAS devices are directly connected to a server, they’re treated like a local disk; the server is responsible for the management of the filesystem. DAS devices can support large amounts of data (100 TB or so), can be clustered (there’s no single point of failure), and are usually high-performance systems. As such, DAS devices are a common choice for high-performance applications. The cost of the device can range from being pretty cheap to very expensive, depending on the levels of scalability, performance, and redundancy that you require.

Although DAS devices are great, they’re limited by the physical hardware. When you reach the physical limits of the hardware (which is quite substantial), you’ll be able to scale no further.

In figure 1 the DAS device would be represented by the storage block, connected directly to the servers.

Storage Area Network (SAN)

Like DAS devices, SANs are also separate hardware disk arrays; they don’t have their own operating system, so file management is performed by the client operating system.

SAN devices are represented on the client operating system as virtual local hard disks that are accessed over a fiber channel. Because you need your web servers to access shared data, the SAN would need to support a shared filesystem. In figure 1, the SAN device would be the storage block, attached to the web servers via fiber channels.

SANs are usually quite expensive, require specialized knowledge, and are rarely used outside the enterprise domain. To give you a clue about how expensive they are, Dell doesn’t even list the price on its website. As for installing and managing SANs, that’s purely in the domain of the long-haired sandal-wearing bearded types. We mere mortals have no chance of making those things work. SAN devices support replication and are highly scalable (they scale much higher than do DAS devices), fault tolerant, high performing, and incredibly expensive. Due to their performance, price, and scalability, this is the solution of choice in the enterprise space. The rest of us can only dream.

Hopefully we’ve justified our earlier premise that implementing a file storage solution today isn’t as easy as it first looks. All the available choices (beyond a certain size) require extensive IT knowledge, skills, and management, not to mention large amounts of cash or a tradeoff between capacity, redundancy, ability to scale, or performance.

This is the state of affairs with regard to the issues with storing files in traditional on-premises solutions. Let’s now look at the Windows Azure BLOB storage service and how it tackles these issues.

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