Network Attached Storage and backup power

They say you should test your NAS in different ways. One of the aspect of such testing is to see how your NAS responds to a power failure. Most interesting cases are where NAS is combined with UPS unit. Just cutting off the power is not sufficient to check an UPS. In normal operation Network Attached Storage receives the messages from an UPS unit constantly and reacts at once.

However, NAS is unable to accept the notifications on power failure during the start up. Additionally, one should consider that the entire cycle (start up then shut down) may require a couple of minutes.

Let's imagine such a scenario:

1. power failure occurs,
2. your NAS unit works on battery for some time and then it shuts down,
3. then power comes back and the Network Attached Storage begins to start up,
4. at this moment power is off once again.

Now the most interesting thing - until NAS starts up it cannot find out that the power failure has occurred.
Once Network Attached Storage loads, it finds out that the power is off and sends a command to shutdown. But it may happen that the system will not be able to shut down correctly since UPS is not capable of supplying power long enough due to battery exhaustion (during the first cycle).

You can avoid such an issue by setting the NAS up in such a way that it shuts down as soon as it gets a message about power failure. All this allows saving battery charge to do one more cycle.

Alternative would be to set the Network Attached Storage up to restart manually, but not in automatic mode after the power failure has occurred.

Complex level RAID100

If you go to Wikipedia you note that RAID 100 is categorized as the nested array layout. There RAID100 is defined as RAID0 that is built on several RAID10 arrays. Thus it turns out that data stored on the disks in RAID100 is exactly the same as in RAID 10.

 So, it is possible to get RAID 0 from RAID 100 array by excluding one disk in each mirror pair. And one more consequence is that RAID100 recovery boils down to RAID0 recovery given that you can get RAID0 form the member disks of RAID10 arrays.

Difference between RAID5 and RAID 5+0/RAID6 and RAID60

Let's consider what is common between RAIDn and RAIDn+0 and how do they differ?

• given arrays are the same with regard to fault tolerance;
• RAID 5 and RAID 50 can withstand only one member disk failure;
• RAID 6 and RAID 60 can survive two disks failure supposing we talk about a 100% probability.

RAIDn+0 arrays have more chances to survive more than one (in RAID50) or two (in RAID60) member disks failure, although not a 100% probability. You can estimate survival probability for various RAID layouts at www.raid-failure.com.

• During the rebuild, RAIDn+0 delivers greater performance as compared to RAIDn which is good.
• RAIDn+0 overhead to provide fault tolerance is more than for RAIDn arrays. RAIDn+0 overhead is one disk per each RAIDn disk group.

Let's make a conclusion - RAID n+0 should be used in case of a large storage while RAIDn - in small (I mean number of disks) arrays.

One of the basic data recovery rules is to verify the recovered data

Most data recovery tutorials insist on verifying the recovered data, but usually most people ignore it. So the following scenario may happen:

1. you recover files and the folder tree seems good enough,
2. the disk is formatted or disk data is destroyed some other way,
3. then you see that the restored data is not recovered properly.

It is evident you need to restore disk content once again. Unfortunately, the original hard disk content does no longer exist.

To sum up the above - it is needed to verify the extracted data first and only then begin to fill the original disk with new files.

When you should benchmark your hard drive?

If you think that your storage device is slow, first step is to check linear read speed, disk access time, and IOPS for the device.
Any benchmark software can give you all the mentioned performance characteristics. Searched the web, I saw both paid and free tools to benchmark data storage devices. I used non paid and simple benchmark software - BenchMe - which gives linear read speed in real time. The free benchmark tool also produces a chart with distribution of access time measured for the data storage device.

On top of that, BenchMe free benchmark software provides a list of features the data storage device supports. It is worth to visit the vendor site to have a look at sample benchmark charts for RAID arrays, SSDs, and hard drives. The only minus I 've come across is that the tool doesn't handle hard disks connected via USB.

www.un-erase.com

Finally online. Behold www.un-erase.com.
Still missing adverts, hehe.

RAID 1 array is able to improve average access time

In theory at least, RAID1 can improve average access time (but not random access time) during the read.
This defies widely-held belief that RAID array cannot reduce access time.

RAID1 has two disks with two identical copies of data. If read operations are organized as follows:

• first member disk serves the read operations to the first half of the disk,
• the second disk handles the second part of the data,

then the average length which a read head should travel to locate a sector is reduced by the factor of two.
Based on the above it is not possible to improve rotational latency. In any case, performance increases since less head travel is needed.
In RAID 1 there is no improvement on writes since both member disks should be synchronized.