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NAS Basics (Data Protection, Performance, Configuration and more)
PART 1
Overview
Back in 2007, I put this little guide together after spending a good few weeks researching a NAS purchase myself. I have picked up much of the information from other forum members posts, so many thanks to them for their help, also lots of different websites and reviews of NAS devices. I want this post to be organic and grow or change as appropriate. So if you have any comments about any of the content, want to add anything, change anything or see any glaring errors, please let me know and i'll update accordingly.
Finally, i hope you find this post useful and that it will save you time digging around finding it out for yourself. I wish there was something like this before i started looking!! 
NAS/DAS Basics
NAS Stands for Network Attached Storage, this is not to be confused with DAS, which is Direct Attached Storage as there are key differences.
First question, do you need a DAS or a NAS? Easy to answer.
Got 1 PC and want more storage directly attached or the ability to take backups? - DAS
Got 1 PC, want more storage/backups plus you've got a home network and see your infastructure growing? - NAS
Got more than 1 PC/device networked and you want devices on your network to see extra storage or a backup method? - NAS
DAS is a storage device that normally connects to either the USB or Firewire port of your PC and it is mainly used by that PC. Yes, the storage on there could be "shared" at the operating system level, but the PC that the device is connected to would have to be switched on whenever the storage is to be accessed by other devices on your network. DAS boxes are also very useful for taking backups to from the attached PC.
NAS devices on the other hand plug directly into your network router or switch allowing all PC's and devices on your network to access the storage at the same time.
NAS Devices are basically a "box of logic" with space for hard disk(s) inside. The number of hard disks depends on your requirements. There are 3 main disk configurations. The logic part providing whatever features your NAS box has, including various levels of data protection.
NAS Configuration Types
There are 3 main configuration types of NAS box, here's a short explanation of them:
1 disk boxes
This the entry level of NAS, but not a recommended method for storing important data. The reason is that you have no resilience or protection from a hard disk failure. If your single hard disk fails, then you lose all the information stored on your NAS. Probably best used for backup purposes only or where data protection is NOT REQUIRED.
2 disk boxes
A much better entry point into NAS. Now you have the ability to protect your data by using mirroring. This is where the logic of the NAS box comes in as the hardware handles the resilience. If you use RAID 1, also known as mirroring, you have 2 drives in the NAS, but one disk is an exact and instantly updated copy of the other so that in the event of a single hard disk failure your data is not lost. You would then replace the faulty hard disk with a new one, the NAS would recreate the copy for you and away you go again.
The downside of RAID 1 is a 50% loss of usable storage as you can only use one of your 2 drives for data, with the other one your protection.
You can also configure these 2 disk boxes as single disks or stripe the data using RAID 0, but there is no protection for your data doing this and it is not recommended if you want to safeguard your data.
4 disk boxes
These devices offer more protection and can also provide better performance. As with the 2 disk boxes you can use RAID 1 protection on the pairs of disks and even RAID 0 (but why would you want pure striping with no protection?).
The major advantage of a 4 disk box is the ability to use RAID 5 which is data striping with parity.(requires a minimum of 3 disks) This means that for performance the data is spread across all drives used, but also, so is your protection (the parity). So in the event of a single disk failure you replace the faulty disk and the contents are rebuilt using the parity data from the other drives.
The only downside to this is the loss of usable storage from 1 of the drives in your configuration. But an acceptable loss to get a good level of protection for your data.
** NOTE ** Protecting your data using RAID is no substitute for a proper backup regime.
Most new NAS drives allow you to connect an external drive to them to schedule backups. You can connect these via USB or sometimes eSATA.
Another option is one of the internet/cloud backups available.
Remember with all RAID configurations that ideally you want exactly the same drives, doesn't have to be the same makes etc, but it helps, but definitely the same size otherwise the RAID array will only be created to the lowest sized disk. So if you've got 1 x 200gb and 3 x 300gb disks for example, you would only be able to create a raid 5 array using 200gb per disk.
For a full description of how the various RAID options work, look here:
http://www.acnc.com/04_01_00.html
http://en.wikipedia.org/wiki/Standard_RAID_levels
Performance
In the main, NAS boxes don't perform as well as single hard disks - FACT.
This was a harsh lesson for me as i was expecting blinding performance from NAS boxes, you just don't get it.
Back in 2007, a single Samsung Spinpoint T166 HD321KJ 320gb drive had an average read speed of 63.20 MByte/Sec (update: in 2010, newer drives around the 90 MByte/Sec mark)
Back in 2007, most sub £1000 NAS devices ran somewhere between 10 MByte/Sec and 40MB/Sec, with between 15-20 MByte/sec the average. (update: medium range NAS can now post figures around 50MByte/sec, whereas higher end units are now pushing 70-100MByte/sec
This link to Smallnetbuilder shows typical read figures for many popular NAS devices when connected to a GIGABIT infrastructure.
http://www.smallnetbuilder.com/component/option,com_nas/Itemid,190/chart,13/
Remember, these figures are for the NAS devices connected to Gibabit ethernet. To get these figures, all elements of the chain need to be gigabit ethernet. So if you are copying a file from your NAS to your PC, the NAS connection, the network switch and the PC ethernet card all need to be working at gigabit. If any part of the link runs slowly, all the other parts will be dragged down to the poor performance levels of that single part.
So if we show you this chart, with is the average read figures of the same NAS devices but when connected to 100MBit networks you can see that none of the devices can get more than 10MB/Sec because the 100MBit network connection is holding the device back. Very Important!!!
http://www.smallnetbuilder.com/component/option,com_nas/Itemid,190/chart,11/
So if you want to get the best out of your NAS and the devices that you want high performance from, ensure that all these elements are all running through a gigbait switch and use gigabit ethernet cards and appropriately capable cabling (Cat 5e or Cat 6)
Also, read access is normally better than write access.
Another point worth making is that even if you have a NAS capable of 100MByte/Sec with full Gigabit networking all along the chain, it still WONT perform as quickly as a single 100MByte/Sec hard disk in your PC. This is because networking (switches/cabling/protocols) add latency to each read and write.
So if you know you only have 100Mbit network or will only be accessing the files over your wireless network, you've got to think is there any worth in spending lots of money on the fastest box out there when the rest of your infrastructure is incapable of using all that bandwidth?
Wireless Device to Gigbabit switch - always 6.75Mbyte max (wireless max drops it down)
100Mbit ethernet to Gigabit switch - always 12.5Mbyte max (100Mbit max drops it down)
gigabit switch to gigabit switch - always 125Mbyte max (full speed allowed all the way through)
So unless you connect both source and destination devices at gigabit,(ie PC and NAS) you are never going to get the best transfer rates out of any NAS. Wireless and 100Mbit connections just strangle the throughput down to the lowest common denominator.
For example, in this chain, your speed will only be as quick as the 'slowest' element.
NAS Hard Disks > NAS Device Capability > NAS Network Card > Switches/Routers > Host Network Card > Host OS > Host Hard Disks.
So if you have a laptop as your host and the hard drive is a small 5400rpm disk that is only capable of 30 Mbyte/Sec, then that's what you'll be limited to, regardless of how good the rest of your kit is.
Another performance issue that has come to light (thanks again ntm1275), this one is round PCI bus limits.
The quote below is from AMD
"The typical PCI bus has a maximum theoretical bandwidth of 133Mbytes and is split between all devices using the bus. In an era of Gigabit Ethernet and RAID controllers, 133Mbytes will act as a bottleneck whether or not PCI uses a shared bus topology"
It basically means that all the PCI slots use 133Mbytes/s, so if you have 5 PCI slots, that would be 133divided by 4 equals 33 each.
I can vouch for this as I had a Gigabit ethernet card in a PCI slot and it was capped at around 30MB/Sec.
A PCI express (PCIe) can run at 250MB/s simultaneously in and out and is not shared with other PCIe slots, so theoretically a PCIe x16 (graphics card slot) could reach 4GB/s
http://en.wikipedia.org/wiki/PCI_Express
So if you can find a PCIe network card, it should give a speed increase over PCI models.
Media Streaming
One of the main reasons we are all looking at these NAS devices is streaming media to various devices (HTPCs, XBOX 360s, PS3, squeezeboxes and media streamers etc)
I've done some calculations using figures found on videohelp.com and come up with the following. These are the "maximum" data rates of video for the following formats:
1x CD-ROM Media: (150 KBytes/sec) 0.15 Mbytes/Sec
And obviously, mp3 and other compressed music formats are less demanding than standard cd-rom material.
Max SD-DVD Media: 1.225 Mbytes/Sec
Max HD-DVD Media: 3.67 Mbyte/Sec
Max Blu-Ray Media: 5.00 Mbyte/Sec
Average bitrate 720p mkv: 8000 kb/sec (1.00 MBytes/Sec)
Average bitrate 1080p mkv: 12000 kb/sec (1.50 MBytes/Sec)
This is a great page for data-rate conversion: http://web.forret.com/tools/bandwidth.asp
EDIT: Please feel free to correct me on any of these figures if you think they are wrong.
I must stress that these figures are theoretical MAXIMUM figures for each technology and the normal usage from these formats would be lower. Plus any ripped files/clips are usually re-encodes with substantially lower bit-rates too.
So as you can see, in theory you should be able to stream even 5.00 Mbyte/sec blueray across a 100MBit Ethernet on the majority of the NAS boxes on the smallnetbuilder list.
The problem will come where you want to stream high volumes to multiple devices. Maybe the kids are watching something on one machine from the NAS and you are in the living room watching another media stream. That's when you need to think is the NAS I'm thinking about going to fulfil my requirement now and for the near future? and is my networking upto the job?
Obviously, wireless networking brings all sorts of other parameters into question with both lower and less than consistent connection speeds.
This table shows the Theoretical Megabit per second speeds converted to MegaBytes per second of various connections (thanks for ntm1275 for posting this). It gives a good overview of the various technologies and achievable speeds.
Code: Megabit first figure (Mbit)MegaByte second figure (MB)USB, Low speed (1.0), 1.5 Mbit/s, 0.18 MB/s USB, Full speed (1.1), 12 Mbit/s, 1.5 MB/s USB, Hi speed (2.0), 480 Mbit/s, 60 MB/s Firewire 400 (IEEE 1394), 400 Mbit/s, 50 MB/s Firewire 800 (IEEE 1394b), 800 Mbit/s, 100 MB/s CD-ROM, 1x, 1.2 Mbit/s, 0.15 MB/s CD-ROM, 52x, 62.4 Mbit/s, 7.8 MB/s DVD-ROM, 1x, 11.1 Mbit/s, 1.3 MB/s DVD-ROM, 16x, 177.3 Mbit/s, 21.1 MB/s Blu-Ray ROM, 1x, 54.0 Mbit/s, 6.75 MB/s SATA I, 1200 Mbit/s, 150 MB/s SATA II, 2400 Mbit/s, 300 MB/s54g wireless, 54 Mbit/s, 6.75 MB/s108n wireless, 108 Mbits/s 13.5 MB/s10Mbit LAN, 10 Mbit/s, 1.25 MB/s100Mbit LAN, 100 Mbit/s, 12.5 MB/s1000Mbit LAN, 1000 Mbit/s, 125 MB/s
Listed here on Wikipedia is a far more exhaustive list of equipment, standards and speeds: http://en.wikipedia.org/wiki/List_of_device_bandwidths
The above speeds are only theoretical speeds that the devices "should" be able to reach, however many struggle to reach these speeds.
Part 2 of this Sticky continues in the next post. |
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2-12-2019 04:28:02
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