What are the ramifications of using NAS (Network Attached Storage) instead of DAS? Will I want to work directly off of it, or keep work locally and only use it for backing up? The answer will vary with the nature of the work, I think.
Let’s start with simple file copying. This was performed using the command line, not the File Explorer GUI.
17 files totaling 273.6 GiBytes
|Windows (CIFS) share||49min 41sec||94 MiBytes/second|
|different drive on same machine||48min 2sec||97 MiBytes/second|
14,403 files in 1,582 directories totaling 2.53 GiBytes
|CIFS share||64min 53sec||683 KiiBytes/second|
|same drive||3h 25min||216 KiBytes/second|
|different drive on same machine||56min 50sec||780 KiBytes/second|
For large files, the transfer rate of 94 MiBytes/second (or 98.6 MBytes/second) is respectable. Everything I could find on real-world speed of Gigabit Ethernet is outdated, with home PCs being limited by the hard drive speed. Note that I’m going through two cheap switches between the test machines.
The small-file case is two orders of magnitude slower! This bears out the common wisdom that it’s faster to zip up a large collection of files first, and then transfer the zip file over the network, and unzip on the receiving side.
I think that the speed is limited by the individual remote file open/close operations, which are slow on Windows, and the network ads latency if this is a synchronous operation. The DAS (different drive on the same computer) is only 14% faster then the NAS in this case. The data transfer time of the file content is only 0.7% of the time involved. The real limiting factor seems to be the ~4 files or directories processed per second. That does not sound at all realistic as I’ve seen programs process many more files than that. There must be some quantity after which it slows down to the observed rate. Since it is similar for DAS and NAS, It must be a Windows problem. I’ll have to arrange some tests using other operating systems, later.
Working with Files
This is what I do all day. What are the ramifications of keeping my work on the NAS, as compared with other options?
Compile Build Job
|Project located on local drive. (A regular directory or a VHD makes no difference)||4min 9sec|
|Project located on NAS, accessed via CIFS (normal Windows share)||10min 29sec|
|using NFS share instead||N/A|
The Microsoft Visual Studio project reads a few thousand files, writes around 1500, reads those again, and writes some more. When the project is located on a local drive, the CPU usage reads 100% most of the time, indicating that the job is CPU bound.
When the project is located on the NAS, the situation is quite different. Given that the actual work performed is the same, the difference in time is due to file I/O. And the extra I/O takes more time than the job did originally; that is, the time more than doubled. It was observed that the CPU utilization was not maxed out in this case. The file I/O dominated.
The same job was performed again immediately afterwards, giving the computer a chance to use cached file data it had already read recently. That made no difference to the time. It appears that with Windows (CIFS) shares, even on Windows 7 (the sharing protocol was significantly reworked as of Vista), file data is not cached in memory but is re-read each time it is needed. That, or the “lots of small files speed limit”, or both, kills performance.
I tried to repeat that using a NFS share instead of the CIFS share. However, I could not get it to work at all. The Windows machine could see the file names and navigate the directories, but could not read any file.
Encoding a video entailed reading one very large file and writing one smaller file. The process’s performance metrics indicate reading only 2.5MB/s and writing merely 80KB/s. I would not expect it to matter if the input file, output file, or both were on the NAS.
Likewise, video editing and programs like Photoshop will read in the files and maintain the contents in memory or manage its own overflow swap space (which you put on a local drive). It’s harder to do actual timing here, but the impression is that various programs are perfectly responsive when the file are directly attached. If that changes when using the NAS instead, I’ll note the circumstances.
All of the performance characteristics above are made with the assumption that the storage unit and the network links are all mine for the duration of the test. If multiple people and pets in the household are using the NAS, you have the added issue of having to divide up the performance among the simultaneous users.