FreeNAS is worth a look for recycling your old computers as Network Attached Storage.
Accessing files across a network is limited by network speed. Network Attached Storage, NAS, makes sense for backups an other less active data. Using a couple of old computers for back makes sense, especially if you have one computer at another physical location. Whatever way you want to use NAS, FreeNAS is a free software+operating system package to turn a computer into NAS.
FreeNAS has a lot of similarities to Linux and Unix because FreeNAS is based on freeBSD, a BSD style Unix. If you use Unix for your other servers, you would start writing your own NAS software using Unix and create something like FreeNAS. For those people who have not suffered through the installation of Unix, FreeNAS is not for you.
Well, it may not be for you. It is free to try and it may work on your hardware. The latest FreeNAS works on one of my older computers. Unix does not have the range of hardware support you find in Linux and there are so many versions of Linux that support on one unix can take a while to propagate to other versions. Just in the limited range of BSD Unixes, there is FreeBSD, NetBSD, and OpenBSD. NetBSD is used by Apple and should see some hardware chip support for Apple branded hardware, which means support for the same hardware chips in all the other brands.
I installed FreeNAS 0.686b2 on a number of spare computers and all of the installations failed to create a workable NAS box. Most of the boxes were spare because they were old enough to be too slow or too small but they were still too new for FreeNAS. People with Unix experience know how to hunt down and capture young hardware drivers. I cannot be bothered finding hardware drivers when there are so many choices for NAS.
When FreeNAS 0.7.1 arrived, it worked on my oldest computer, one of the first Athlon x64 Dual Core processors with a motherboard of the same age. A lot of the motherboard chips are switched off including the audio chip.
If you want to use FreeNAS, check their hardware compatibility guide. BSD Unix is really slow at adding hardware support. There are alternatives to FreeNAS based on Linux and some distributions of Linux are way ahead on hardware support.
Did you shudder when I mentioned installing Unix? You probably want something easier to install. Ubuntu is probably the easiest version of Linux to install. You can set up a NAS box using Ubuntu and most other distributions of Linux. I suggest you install Ubuntu as a desktop computer and learn to use Ubuntu then install file sharing options including Samba and FTP. After you work through the various ways of sharing files in Ubuntu, you will understand what you are doing with FreeNAS.
NAS appliances use the latest power saving chips and designs to reduce power consumption. You also save power by consolidating data onto fewer disks. When you use FreeNAS with very old hardware, your main expenses could be electricity and cooling. Look at the disk and computer sleep mode options in FreeNAS to save electricity.
Consider retiring the oldest hardware, especially the disks. Disks have a life expectancy of 5 years. A small disk from 5 years ago might be 200GB. You can now buy a 1000GB disk that uses less electricity than one of the old 200GB disks. 2 modern 1GB disks in a RAID 1 array use far less electricity than the equivalent built on old 200GB disks, which would be 6 * 200GB in a RAID 5 array.
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You need more than one disk for RAID and you can, in theory, use an unlimited number of disks. In practice the hardware limits the number of disks you would use in one array but you can have arrays of arrays to produce very large sets of disks. FreeNAS works on a wide range of hardware giving you a wide choice of disk and RAID configurations. FreeNAS is based on BSD and BSD is slow to acquire drivers for new hardware, making FreeNAS a good way to recycle old hardware as storage but not necessarily the best choice for new hardware.
A RAID array containing a one year old disk and a five year old disk does not give you an average reliability of a three year old disk. Instead you get the reliability of the oldest disk. You also get the speed of the slowest disk. If you are using two generations of disks, consider dumping the oldest disks or creating two arrays with the new disks in one array and the old disks in the other.
A regular workstation tower has slots for 4 disks, 2 * 3.5" floppy disk size slots, and three 5.25" DVD drive size slots. You do not want to waste a disk slot on a disk for FreeNAS so consider installing FreeNAS in a Compact Flash card or a USB memory stick. The initial installation from CD could be via a USB CD drive leaving all three DVD drive slots available for disks. You can then install 7 of your old disks. An old cheap motherboard has connections for 2 PATA and 4 SATA disks. You can buy a lot cost add on card with 4 SATA connections to let you connect all 7 disks.
Update: Consider using an SSD instead of a CF card now that SSD prices are falling. See Compact Flash or SSD for your boot disk?.
As an example, if you had 4 old disks (3 years old) and 3 very old disks (6 years old), you would put the 4 old disk together in the 4 disk slots and connect them through the 4 motherboard SATA connections then make them into a RAID 5 array that you expect to last a long time. The 3 very old disks would go into the 5.25" slots using adaptors and connect through the add on board to make a RAID 5 array. You would use the second array only for short term storage and would expect them to break any day. When one of the very old disks breaks, which could be within the hour or 3 years later, you throw out the whole lot.
Continue with the example. If one of the not so old disks breaks, consider throwing out the 3 very old disks then installing three brand new modern big lower power disks, create a new RAID 3 array, then copy everything from the old disk array to the new array, then reconfigure the remaining working 3 old disks as a new RAID 5 array.
If your disks are less than 5 years old, it is worth recycling them in smaller arrays as individual disks break because the breakages should be rare. When the disks are older than 5 years, you can expect breakages so frequent that maintaining the arrays is not worth the effort. After you learn how to create a RAID array using FreeNAS during that initial installation, you can look forward to the shock of recovering from the first disk failure. Read up on the difference between RAID 1 and RAID 5. Make sure FreeNAS is set to notify you the instant a disk breaks because a second disk breakage is highly likely at the same time.
FreeNAS could be a great way to recycle old computers as NAS boxes if you have time to learn how to use FreeNAS and have a compatible computer. Linux users might find it easier to convert their favourite Linux distribution to a NAS system and they will have a wider range of hardware compatibility. Windows users will find Ubuntu the easiest Linux to learn and a better starting point to build their first NAS. If you do not have the time to install software, use a prebuilt NAS box but do not expect fast access from anything offered at a reasonable price.