In our last article we explained the basic idea of RAID, as well as the most commonly used terms when talking about this type of storage. In this second part, we’ll take a look at what RAID levels are and explore how some of the traditional level configurations work. We’ll also find out what challenges RAID storage can bring if data recovery services are required. Let’s take a look!
RAID storage levels
First, let’s delve into the three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Different RAID setups use one or more of these techniques, depending on the system requirements.
Standard RAID configurations such as these are referred to as levels. There were five levels originally created, but many more variations have evolved, notably several nested levels and many non-standard levels (mostly proprietary). Already we have seen RAID levels expand from RAID 0 all the way to RAID 51 (and beyond). Different levels have different types of redundancy, and a trade-off usually has to be made between fault tolerance and performance, depending on the application. For example some basic levels include:
RAID 0 – Often called ‘striping’ and is the most basic RAID level. Offers no redundancy but excellent performance. Data is striped across at least two disks and with every disk added, read/write performance and storage capacity is increased over a single drive.
RAID 1 – This level is also called ‘mirroring’, which (as the name suggests) mirrors the same data across two disks – providing the lowest level of RAID redundancy. This level offers up to double the read performance over a single drive, but no increase in write speed. Stored data is always accessible as long as one disk is still working.
RAID 5 – This is a common configuration that offers a decent compromise between security and performance. It requires at least three disks and provides a gain in read speeds but no increase in write performance. RAID 5 introduces ‘parity’ to the array, which takes up the space of one disk in total. This level can tolerate one disk failure.
RAID 6 – This takes the concept of RAID 5 further – a minimum of four disks are required and dual-parity is introduced, meaning data can be recreated even if two disks fail within the array.
Modern RAID arrays
Nowadays there are many different ways to get more out of your RAID system. However, given the highly complicated and technical nature of modern arrays (and how they can be utilised with other complex systems for significant efficiency and cost benefits, such as virtualisation), it is not uncommon for one of these technologies to suffer a fault. When this happens, due to the interconnectivity of multiple systems this can potentially cause significant data loss, which can cost businesses millions in downtime.
Modern RAID arrays can also utilise multiple file systems, like BTRFS or ZFS at hardware level, with NTFS or HFS layered over the top for application support via virtualisation.
Challenges for data recovery
As you may have gathered by now, RAID arrays are highly complex. This is often intensified within enterprise IT infrastructures, as RAID systems are used mostly for business critical applications; where availability and efficiency are crucial factors. What’s more, add-on technologies like virtualisation or database applications can spell disaster for a business if the system was to fail.
From a data recovery perspective, it would usually be necessary to not only reconstruct the RAID file system and bypass any physical failures, but to also assess any virtualised architecture that may exist. This can often make a recovery attempt extremely complex and time-consuming; however in many cases recoveries can be very successful.
Unfortunately drives can (and will) fail at some point in their lifetime. When this does happen, if individual drives fail then (assuming it is a RAID 1 or greater) the faulty drive can just be replaced with a new one and have the data storage map rebuilt with zero data loss. Though if a drive failure exceeds the redundancy capacity of the RAID you should consult with a data recovery specialist for the best chance of recovering your data. It is imperative to make sure that your chosen provider has the tools and expertise to recover from any configuration or data loss situation. You should also assess whether or not they have direct partnerships with storage vendors and development capabilities for accommodating bespoke configurations.
Mikey has been with Ontrack for 6 years and is the Partner Program Manager, based out of the Epsom, UK office. He is a regular contributor to the Ontrack blog, usually writing about how different types of data storage technologies work. Outside of work you’ll find him either with a guitar in his hands or learning about rocket science.