Friday, August 26, 2011

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

  • 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.

Wednesday, August 10, 2011

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.

Wednesday, August 3, 2011

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.