Shared File Systems: Why Pay More?
The cost of NAS devices, particularly with SATA drives, is often far less than the same amount of storage in standard Fibre Channel technology, and sometimes even significantly less than Fibre Channel RAID with SATA drives. Add to this the cost of the shared file system, and the difference becomes huge.
There's also the issue of complexity. With NAS, you can manage a great deal of storage with just a few keystrokes, and that storage can be attached to many systems over a TCP/IP network. By comparison, shared file systems often have limits on the number of hosts you can attach, and the complexity of administration is almost always significantly greater.
So why doesn't everyone just buy NAS storage and be done with it? The decision to use a shared file system or a NAS system comes down to two considerations: your budget, and your requirements.
This is the first in a two-part series comparing NAS devices to shared file systems and Fibre Channel RAID. In this article, we'll compare the cost of NAS and shared file systems, and next time we'll examine performance issues. The environment we are going to use for comparison is one that requires multiple servers to access the same data. We are not looking at the NAS environment where a server sees only the NAS device as local storage space for that server only.
The total cost of ownership for NAS and shared file systems can be calculated by determining the following:
- The initial cost of the hardware and software;
- The cost of equipment and software to backup the system;
- The cost of maintenance;
- The cost of incremental upgrades or increases in storage space, and
- The cost of training and staffing.
Hardware and Software
When buying a NAS device, you're buying X amount of storage, and that usually includes the cost of the NAS head and the software to run the NAS device. You sometimes pay extra for things like dual failover heads, multiple interfaces, the size of the read and write cache and assorted other features, but you buy a single package. You usually use your existing IP infrastructure to connect the NAS device to the systems that you want to access the NAS storage.
If you buy a shared file system, you need to buy the software (the file system), the Fibre Channel HBAs to connect to the SAN, Fibre Channel switches, and, of course, the storage. For the storage, you have all types of choices, from low-end Fibre Channel RAID using SATA devices all the way to high-end enterprise storage with Active/Active connections, Fibre Channel disk drives and a shared cache that has a very low failure rate.
One final consideration is that with NAS, the amount of CPU required to move the data is often far higher than with standard SCSI. The amount of overhead depends greatly on the file system and connection topology, but this might result in more CPU needed on each server to get the required I/O performance.
Backup Equipment and Software
My use of the term backup is generic, be it remote replication or traditional tape backup. That said, I strongly believe that you must at some point get important data on tape to an offsite location in case of a disaster or mishap.
Backup should be included in the initial cost. Many NAS devices have a backup option that you can buy with the hardware. Sometimes these devices are low-end tape drives that are slower than your current backup system, and other times you have the option to buy more expense hardware, which is often the case with higher-end products.
One issue is that you sometimes need to buy additional hardware or software to integrate or duplicate the NAS into your environment. With shared file systems, especially some of the higher-end file systems, backup is accomplished via an additional purchase option for HSM software.
The cost of support for NAS and basic SAN hardware is generally a function of the hardware costs, so NAS solutions are often less expense in terms of system maintenance. If your RAID and NAS devices are both using SATA drives, the cost might be very similar, and the RAID might even be less expensive, since in effect NAS contains both RAID and the CPU to run the NAS. The cost of maintenance of the shared file system must be added to the hardware costs.
Some other maintenance costs that can get lost in the shuffle include the cost of backup software and hardware (tape drives, robots, tapes) and the cost of downtime for backups and outages.
Backup hardware and software are bundled with some NAS products, and some tape drives might have better compression than others (see Back to the Future with Tape Drives for some background on tape drives).
Downtime has a cost in some cases. If you are running a credit card approval code operation, then every second of downtime costs tens of thousands of dollars. If you must have the hardware unavailable to do a full backup, then you might have a cost associated with the downtime for backup in certain environments. I include this in the cost of maintenance, but it could just as easily be part of your requirements. Either way, it is still a cost that must be considered when looking at the total cost of ownership.
I have heard users say hundreds of times that they won't need to upgrade their systems, that they have all the disk space they could ever need. My response is always the same: Yeah, right! It has been said that the universe expands to fill empty space, and the same can be said for storage space and any other space — if it's there, it's going to get used.
The key areas that need to be reviewed for this cost are:
- The cost of increasing storage;
- The cost to the organization in downtime to increase the capacity, and
- Whether the file system can grow dynamically or also requires downtime.
Training and Staffing
Training for management of NAS devices, even high-end NAS devices, is far less difficult than training for a RAID, Fibre Channel switch and shared file system, and possibly for HSM software too. With NAS, you do not need a specially trained staff of SAN and shared file system experts. Running a NAS system is far less expensive than an equivalent shared file system.
And Now for Requirements
Understanding your requirements will help you decide which is the best choice for your environment. The important requirements as I see them are:
- Performance needs;
- The size of the storage system, and
Determining which is a better choice, a shared file system or NAS devices, should not be a difficult decision today. The tradeoffs are pretty clear, and stepping through the process is not that difficult.
The problem is that file systems as we know them have been around since about 1965 (see Choosing a File System or Volume Manager), and we need a paradigm shift to make NAS and shared file systems scalable and high-performance. I believe that shift is coming. There is a new standard that is been put forth by the T10 group (www.t10.org). This standard is called Object-Based Storage Devices, or OSD (see A Storage Framework for the Future). If the standard is adopted a few years from now — and I sure hope it will be — file systems and storage devices as we know them will change, and NAS and shared file system concepts will merge.
In this article, we have covered the cost issues for NAS and shared file systems, and as you can see, there are many of them. In the next article, we will focus on the requirements surrounding performance issues and draw some conclusions about the tradeoffs you will face.
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