Linux and fashion

All users know that Linux developers like and support in every way the diversity of versions which support various incompatible things whether they are really useful or not.

It is enough to look at the md-raid superblock that is known to have at least three "just a little" different versions. Most of them differ only in a location of a superblock in relation to the array contents. Such a difference can result in several unexpected consequences, e.g. you want to recover NAS manually following the instructions at www.nasrecovery.info, but Linux is not able to assemble NAS because it is not able to see a single superblock.

In my opinion, those who develop Linux are just a little mad at a location of a superblock. As a consequence of all this - with enviable regularity another Linux version with a new superblock location is released.

Want to build an array?

If you are going to make some big storage, be aware of the following concerns:

• needed array size
• redundancy
• performance
• required ability to boot from the RAID array
• money you are going to spend.

Taking into account the items listed above, then you need to decide what solution fits for you - create your own storage or just order off-the-shelf storage.

Should you choose the latter option, then have a look at this one of RAID Tips. If you decide to create your own RAID then go to www.raidtips.com.

URE values - real or not?

When reading technical specification of disks given by vendors it can be noted that often makers provide not real Unrecoverable Error Rate values. This URE probability value is widely utilized to substantiate naive statements similar to "RAID 5 is dead by 2009" and to guess chances of double failure in RAID5. These calculations get people building their own RAIDs concerned.

In fact, the vendor URE data seems to be very far off the mark. Read technical documentation on Hitachi official website, they have kind of interesting URE values for 3 TB hard disk - 10^-14 errors per read bit. Such a value can be converted to the probability to read the drive from the start to the end and not encounter an URE is:

(1- 10^-14)^(8*3*1012)~0,79

therefore, the probability of the disk failing to read one sector is about 20%.

In other words when you have a disk filled at capacity there is a non-negligible chance (about 20%) that you are not be able to get data back off it. This is easily proven wrong by simplistic testing.

Random access time in RAIDs

The basic characteristics of a data storage speed are:

• access time which is defined as time delay between when a request is addressed to a storage device and the moment the requested data begins to come in. 
• throughput is sustained average transfer rate.

It is well-known that RAID level 0 increases throughput (N times increase ideally). When one plans to build a RAID array, only the throughput numbers are considered, not thinking about access time.

Access time in a regular disk includes the time to position a read head above the track (so called seek time) and the time which is needed a drive to bring a sector under a read head (so called rotational latency). No matter how many member disks are in RAID 0, there always exists such a sector which is simultaneously the furthest away from the head and this sector is not contained in the cache. For this sector the access time will be the same as in case of a single drive. The only way to decrease access time is to stick to an SSD.

P.S. One can easily estimate access time (and other performance characteristics) using free benchmark software BenchMe.

New free benchmark tool

I wanted to check my RAID from the performance point of view and so downloaded three benchmark utilities:

• HD Tune Pro
• Crystal Disk Mark
• BenchMe

 The price of HD Tune Pro is $34.95 and it provides linear read speed, access time, and quite a few other metrics most of which are not significant. I say these things because I just don't know where I could use this data. One more disadvantage is that it was difficult to find such a main performance characteristic as IOPS.

 CrystalDiskMark doesn't have an option to benchmark a physical data storage device, but is limited to partitions only. From my point of view this might be a drawback. The tool gives read/write speed for various number and size of requests. You can also get benchmark either for linear or random read/write speed.
All tests are designed in such a way that one is able to vary amount of data to be read.

And finally meet one more benchmark tool, BenchMe, that is a really easy-to use benchmark software. Unlike CrystalDiskMark, it benchmarks physical devices, but not logical volumes. Using the software one can get the various performance parameters such as linear read speed, distribution of access time, and a list of features. IOPS (I/O operations per second) values are measured for queue depth 1 and 32.

 Thus, the conclusions of benchmark software review are:

• HD Tune Pro is a paid software with a lot of useless benchmark parameters.
• CrystalDiskMark is a free tool which is characterized by inconvenient interface and with no ability to scan physical data storage devices.
• BenchMe is a free program that gives only really needed benchmark parameters in the most convenient representation.

There exists a time-tested Data Recovery Law - always verify recovered data.

 Many of the data recovery tutorials advise users to verify the retrieved data, but usually most people don't do it. So the following scenario may take place:

1. you extract files and documents and folder tree seems reasonably correct.
2. the disk is formatted or disk content is destroyed in some other way.
3. then you realize that the retrieved data is not recovered correctly.

It is evident you need to recover disk content once again. However, the original disk content is no longer available.
Moral of the the above is that you should verify the recovered data first and only after checking start to fill the original hard disk with new files.

"You need to format the disk in drive X: before you can use it."

It may happen when you disconnect the storage device (flash drive) without using "safely remove hardware" option, then, when you connect the drive the OS will say "You need to format the disk in drive X: before you can use it.".

Typically such a behavior means that you have a RAW file system. Obviously, initial filesystem on your drive was corrupted because the data stored in the buffer during disconnection of the drive was lost due to inappropriate disconnection.

You can quickly solve the problem by simply formatting the drive, but be aware that the data would be gone forever according to the format level you use. If the data is important you need to retrieve data first and only then format the drive. Data recovery in case of RAW filesystem is like unformat and easy to do - just download some data recovery software.