Make a software RAID bootable?

Is it possible to create a bootable software RAID0, RAID 5, or span with a Windows installation?
The answer is "NO".
One cannot start an OS from a software RAID0, RAID 5, or JBOD. A hardware controller is needed to do this.
It is not possible to boot off the software RAID for the RAID is not accessible until the operating system is done loading, and the operating system itself is on the RAID. The only option is to boot off a software RAID 1, and even that is not that simple. To boot from a "shadow" disk of a software RAID1, you probably need to copy the master boot record first.

One of the main data recovery policies is to check the restored files

When recovering data, just a few people would make sure that the data has been restored correctly although such a check is recommended in most all data recovery tutorials (e.g. in the data recovery guide).

Such a neglect may lead to the following scenario:

1. you retrieve files and documents and the folder tree seems reasonably correct,
2. the disk is formatted or original disk data is lost some other way,
3. you find out that the retrieved data is not recovered properly.

It is obvious you should restore data once again. However, the original data is no longer available. So the conclusion is that you should check the retrieved data first and only then start to write data on the disk from which you have extracted data.

It is interesting that the same approach should be used in many other situations. For example, you bought a NAS device and the documentation says that it has an UPS and therefore can withstand power failure. But you should not just believe this but test - before placing data on the NAS, turn off the power and  see how your NAS feels in case of a power failure.

Do-it-yourself RAID tips

When planning a new RAID, you would want some factors straight.
 
Protection against drive failures.
Keep in mind that RAID is not a substitute for a proper regular backup. RAID can not save you from user errors or natural disasters like flooding, earthquake, tornado, or whatever is common in your area.

Anyway, some RAID types will continue working even if one of the array's drives stops working.
These levels include RAID1, RAID10, RAID 4, RAID5, RAID 6, and exotics like RAID5E, RAID 5 EE, and RAID DP. It is discussed that RAID5 with a large number of member disks is unreliable, using calculations based on vendor-specified URE values, which can be shown to be incorrect. 

Capacity.
The RAID array size limited by

• the size of a single hard drive,
• maximum number of disks the RAID controller would handle,
• and the overhead needed for redundancy, if you choose to use a fault-tolerant array.

Should you need simple array size calculations done for you, have a look at this free RAID Calculator.

Performance requirements.
Of all the fault-tolerant RAID types, those not using parity, namely RAID1 and RAID10 are the preferred for random writes. If the RAID is almost always used for reads (for example a static HTTP server), or as a write-only storage (a backup), then RAID levels using parity, namely RAID5, RAID6, and variations thereof are OK.
If fast random writes are needed, use RAID 1+0.

To quickly learn about performance, price, and redundancy for various array types, look at the "RAID triangle".
Keep in mind that RAID doesn't decrease random access time. For good access times, use Solid State Drive.

Difference between complex layouts - RAID5 and RAID 50/RAID6 and RAID60

Let's compare these RAID levels:

• these RAIDs have the same fault tolerance, however in RAIDn+0 arrays the chance to survive more disks failure is greater (but not 100 %). You can easily estimate and compare probabilities to survive multiple disk failures for different array layouts using free RAID failure calculator. 
• RAID5 and RAID50 survive one disk failure with 100% probability,
• RAID 6 and RAID6+0 can withstand two member disks failure.
• When rebuilding, RAIDn+0 shows greater performance as compared to RAIDn which is undoubtedly better.
• RAID n+0 overhead increases with the number of RAIDn disk groups contrary to RAIDn where the overhead is fixed and does not increase as the number of disks increases.

Thus, RAIDn is more appropriate variant for small (in terms of number of disks) arrays, while RAIDn+0 is ideal for large data storage.

Do you know that RAID 1 is not as reliable as you think?

When using RAID1 it is required to access data on the second disk as well. If you do not, then one or even several bad sectors may develop on the second member disk of the RAID 1.

In this case the RAID 1 does not have fault-tolerance any longer and on top of that you do not know about it. Usually this situation occurs if the RAID 1 is idle most of the time. To make sure there is still fault tolerance you need to read data from two hard disks and compare it. Heavily loaded arrays don't have this problem.

It can be shown that in RAID1 used by Windows data is typically read from the first disk. This quirk renders Windows RAID1 unreliable.

It is interesting that hot spare drives are also subject to the same problem; therefore they are needed to be tested as well.

What are the chances to recover a NAS device using Linux OS?

At www.nasrecovery.info you are offered to use Linux as a first thing to recover NAS. In practice, it does not work at all or at least you cannot obtain any meaningful results.

Let's give an example - sometimes happens that you assemble an array by means of md-raid driver in VMware only to see that one member disk has dropped out after restart. Although it is well-known that VMware provides infinitely reliable virtual hardware, so the only option remains is that md-raid is buggy. Thus, there is very little chance to assemble the array with valuable data with the help of Linux. Keep in mind, that it makes sense to try at least several different Linux versions for, as of now, quite a number of Linux distributions are available at the official Linux web site.

By the way, some of you will be lucky, although it is not mentioned on www.nasrecovery.info - some NASes do work on Win Embedded.

NAS and backup power

I saw at www.raidtips.com an interesting post "Test your RAID" where it is recommended to pre-test fault tolerance.  One of the suggested approach to test fault tolerance is to do a simulated power failure.
However, just turning off the power is not enough to test an UPS. In normal operation NAS receives the notifications from its UPS unit continuously and takes action immediately. However, when starting up the reports concerning power failure are not received. On top of that one should consider that the entire cycle (from start up to shut down) may require up to ten minutes.

Let's consider such a situation:

1. suddenly power fails,
2. your NAS unit takes power from the battery for some time and then shuts down,
3. the grid power comes back and NAS begins to start up,
4. at this moment power fails again.

Now NAS is not able to process a report on power failure event until it fully starts up. As soon as Network Attached Storage loads, it detects that the power failure has occurred and starts to shut down.
But it may happen that the system will not be able to shut down correctly since UPS doesn't supply power long enough due to battery exhaustion (during the first cycle).
One can avoid this by setting the NAS up so that it finishes its work as soon as it receives a message that power failure occurs. Doing this one can save enough charge of battery for another start-stop cycle.
Also you can configure Network Attached Storage in such a way that NAS does not restart automatically, but manually after the power failure has occurred.