RAID-6 vs RAID-5 (The Case For RAID-6)

​RAID-5 is the most popular choice for storage arrays today, as it is relatively simple to implement at the hardware level and is able to continue to operate in the event of failure of a single device. For many years RAID-5 has provided adequate levels of protection for highly reliable operation, but the growth in data storage requirements and the corresponding increase in capacity of arrays has the potential to impact the reliability of RAID-5 systems.

It is easy to see that larger data storage requirements leads to arrays with more drives and with more drives comes a higher probability of one drive failing. However, another problem is now becoming apparent - the rate at which an otherwise healthy occasionally drive fails to read a sector from the disk. These errors are known as "non-recoverable read errors" and the rate at which they occur is typically specified by the drive manufacturer as "non- recoverable read errors per bits read". This rate ranges from one error per 10^14 bits (12.5 TB) read for consumer SATA drives such as the ​Seagate Barracuda 7200.10 to one error in 10^16 bits (1.25 PB) read for enterprise grade drives such as the​ Seagate Cheetah NS​.

​RAID-5 Array Rebuild​

​In order to rebuild the array after a drive failure, a RAID-5 array must read the entire contents of every remaining disk in the array. If the array consists of seven 1 TB disks, 6 TB of data would be read during this process, 1 TB from each of the six non-failed disks. If consumer SATA disks are used, one read in every 12.5 TB of data read is likely to fail, so there is almost a 50% chance of an unrecoverable read error during this process. When such an error occurs the RAID controller cannot continue the recovery process and the array will fail. When this happens, all is not lost. Our experienced data recovery experts can rebuild arrays which have failed in this way, often with same-day turnaround.

​Drive failures in a RAID ​array require immediate action. You need fast, secure results, and R3 offers the solutions you need to effectively maintain your data and system after any type of failure. Call us today at 0800 999 3282 to get started with your unique case.

​RAID-6 improves on the reliability of RAID-5 by being able to tolerate failures on two disks simultaneously, thus considerably enhancing the reliability of the array. This enhanced reliability comes at the cost of additional hardware in the form of the additional disk per array, as well as increased complexity in the RAID controller.

In an ideal implementation, RAID-6 is only marginally slower than a RAID-5 array with the same capacity and identical drives. The minor performance penalty comes from the need to wait for data on one additional disk to rotate to the read heads. The complexity of the calculations required when writing data mean that real world implementations are often far from ideal and are limited in performance by the rate at which parity can be calculated, rather than the speed of the disks.

​How RAID-6 Works

While a RAID-5 array contains one additional drive, over and above those required to store the data, a RAID-6 array contains two additional drives. The capacity of an array consisting of N drives with minimum capacity C is thus given by the formula (N - 2) * C. For example, a RAID-6 array of seven 1 TB drives would have a usable storage capacity of 5 TB.

The following diagram illustrates how the data and parity blocks are distributed across disks in a RAID-6 system.

RAID 6 Array

EMC SAN RAID 5 Disaster Recovery Alert

An unknown number of EMC Storage arrays could be a ticking time bomb in the event the data on them is not backed up elsewhere.

An obscure bug in an EMC RAID controller is a rare discovery for any data recovery engineer but the team approach by R3 achieved a recovery that very few data recovery labs in the world have ever seen let alone recovered successfully.

​This is of interest and notable because most Dell / EMC technical / disaster recovery support engineers would just declare the data corrupt and unrecoverable, restore from a backup and not realise it is possible it will happen again.

EMC RAID Storage System in the Lab at Secure House Sheffield

​Cloud Backup Gone Wrong​​​​​​​

​A problem with a cloud backup meant that the only chance of saving months of work from being lost was to call a data recovery company.

R3 initially lost the job to a substantially cheaper / high risk data recovery / computer forensics investigators. The client did subsequent investigations and elected to retrieve the server from the competitors HQ in Wales and drive it direct to Security House in Sheffield.

The 15 Disk RAID 5 configuration included 1 hot swap disc, but 2 of the Seagate FC series fibre channel discs had failed.

The recovery was complicated at a number of levels but in summary:

  • A disc had failed
  • A hot disc did not work
  • Another disc failed
  • The resync to the hot disc failed in some unexplained manner

The discs were formatted 520bytes per sector which was common for enterprise class SAN but data recovery algorithms / software are most stable working with 512byte per sector formatting.

The client contacted a British / Global data recovery company who were unable to diagnose the problem or propose a solution and R3 were called to take over the case.

On the face of it solving the disc failures, imaging conversion, VMFS and recovery of specific data from within just 1 of dozens of VMs was complex enough.

But as the tasks progressed something was wrong and it just did not make sense.

During any disaster / failure there are a number of factors​ - each year I manage several of the larger disasters in UK and dozens of relatively routine for the R3 team recoveries with disc arrays in pools from as little as 4, 6 12, and others with 24, 55, 180 and recently a pair of 240x 4TB SAN with varying RAID combinations and Tables.

But this EMC RAID 5 was different, it not only had a very seriously degraded disk which had sustained a head crash but the resync had written as data not parity and prevented the data from the RAID being accessible.

In fact it prevented a RAID recovery being possible. R3 engineers were initially stumped because the last disc to fail was proving difficult to get a full image from and took a few days in itself to be recovered and its image converted to 512bytes per sector.

The first to fail disc initially was ruled out as one of the discs being needed because it was out of sync by several weeks.

But on rebuilding the RAID5 volume the VMs were showing as corrupt and all involved including the developers of RAID recovery software and hardware could not help and hit a dead end.

Later we realised just how unique this EMC failure was and that no one had identified a similar case ever.

EMC now part of Dell is "fairly good gear" R3 have worked on a number EMC data recovery cases where multiple disc failures have caused the data to be inaccessible but non that effectively wrote back to a RAID5 disc member in RAID0.

All involved had never seen anything like this on an EMC or on any hardware for that matter.

After solving this a bespoke system configuration was built and a script developed. All VMs were extracted in order of priority and the contained data extracted and tested by the client.

Apart from the normal server dirty shutdown problems needing some database file repairs it was a full recovery.

A Firmware Affair (RAID 0 Case Study)

Firmware issues

Sometimes it is very difficult to correctly diagnose a drive: the symptoms one is facing can be confusing and can lead you in the wrong direction.

We received a RAID 0 system, made up of 2 drives. One of them was completely unresponsive, it had no sign of life when powered on.

In such situations, the first issue that comes to mind is a major PCB failure; so you have two solutions: replace it with a matching PCB that you know works, or try to repair the damaged board.

Well, this time the PCB swap didn’t help at all.

We checked all the other components and they were fine; at the end of a long process we found that the problem was in the ROM chip: the firmware had suffered a severe corruption.

The word Firmware is related to a collection of small programs, this is what “controls" the hardware to put it simply. Usually, such microprograms are stored mainly on a Read Only Memory chip and partly on the platters.

Nowadays almost all modern intelligent devices have a firmware onboard that oversees the proper functioning of the device.

Maybe you have experienced it with your digital camera or mobile phone: A Firmware upgrade results in a totally new behaviour (hopefully better) of your device.

Well, in our drives case, the FW damages prevented the PCB working at all.
Here come the real issues. Every Hard disk firmware stores some unique information, belonging to that specific drive, vital to its correct functioning: you cannot simply take a FW from another drive and overwrite it.

So, we started a real handcraft job trying to reconstruct the damaged FW modules cutting and sewing from other drives.

Our efforts have been rewarded: the customer was very pleased to be able to have his data back in fully working condition!

R3 Data Recovery is real lab that deals with real disasters each an every day. If you have RAID that has failed or any data storage device, we are the people to contact. Call us today on 0800 999 3282 for immediate help and assistance.

How To Identify RAID Devices

Synology RAID storage device

Modern storage devices come in many forms from simple USB devices that can be transported around in your pocket all the way up to network attached devices that sit next to your router or in your office accessible from your network or over the internet.

This is fine until you need data recovery and you are asked the question "is it a single hard drive or RAID device.." obviously this effects the cost of recovery.

Identifying these devices isn’t always straight forward but I hope this goes some way to explain the difference.

First of all there are a handful of vendors that regularly make multidisc RAID backup devices:

Buffalo, LaCie, NetGear and Gstore to name a few;

These devices can be large and have options on the rear of the unit for safe or large storage which is a giveaway that it’s a multidisc RAID device. The unit is heavy and bulky with lights on the front to indicate disc activity also may have the facility to remove the hard drives to upgrade the storage capacity in the future and also hints that it may be RAID.

Remember if all else fails just a simple Google search on the model will bring up the manufactures details and help guide you through some of the questions you may be asked by the data recovery company.

If we know this it can sometimes make the engineers lives easier as some manufacturers will tell you on their websites the parameters fo the model you may have. This could possibly save hours o time trying different parameters and configs and you could have your data back faster than you thought.

R3 Data Recovery is real lab that deals with real disasters each an every day. If you have RAID that has failed or any data storage device, we are the people to contact. Call us today on 0800 999 3282 for immediate help and assistance.

FreeNas RAID 5 Server Emergency Recovery (Case Study)

​The client needed help as his freeNas RAID 5 had a set of disks (4 disks) that had been formatted and needed assurance that the data will be recovered. Naturally we said that it’s most likely that we will recover the data but there may be a chance that we won’t be able to locate the data.

The client was still willing to pay the full payment before we had even started the recovery, which meant he had his complete trust in us and our work.

FreeNAS RAID5 hard disks set
  • Thursday 11:54am - Customer sending in NAS
    We agreed that we needed to get the NAS into the lab as quickly as possible so since the company is based in Rotherham, the client suggested that one of his staff members should drop the NAS off to us from Rotherham to Sheffield.

  • Thursday 16:22pm - Received NAS
    Once payment had been sorted out, one of the employees of the client arrived and dropped off the NAS. He then proceeded to give us all the information that we may need to help us with the recovery, he also filled in what his boss wanted as priority data just so that we could concentrate on that rather than the whole amount of data to speed the process along. We than booked in a took photos and noted all serial numbers of each drive.

  • Thursday 17:26pm - Diagnosis and Cloning
    Due to the fault being an issue of the drives being formatted, there was nothing wrong with the drives, we did some firmware tweaking for safety and began to clone the drive as quickly as we could.

  • Thursday 17:45pm - Client ​Advised
    At this point the client was advised that the recovery was under way and that we were cloning the drives. We also advised that there shouldn’t be anything wrong with the drives since the drives were just formatted and that there was nothing wrong with them mechanically.

  • Friday 10:00am - Cloning Complete
    All the drives had been patched and cloned 100% with no bad sectors. It was just a case of scanning the drives to check if we can visually see the data after the scans. It is also worth noting that the customer had used Recuva software on drive 1, so we concentrated on all 4 drives but we looked out for if anything was affected on drive 1 because of the Recuva software. Running data recovery software, yourself can work, the software may be able to find the data that you are looking for, but if you are unaware of how data recovery works and you begin to save your lets say deleted data to the same drive you’re recovering from this could lead to many problems as you may be possible overwriting the data you are trying to recover, which would make our job increasingly difficult and most likely impossible to do. This is why we always advice our customers not to run data recovery software as it could make the situation a lot worse than it actually is.

  • Sunday 02:04am - RAID configured and Data extracted to a 4TB external drive
    From Friday to Sunday, the transferring process was pretty straight forward, the priority data from the RAID was found after the scan and we managed to copy the data to a 4TB external drive ready for the customer to pick up Monday morning. Admittedly it was pretty tight in regards to time, as we were extremely close to the emergency service, obviously it didn’t help that we had to transfer over 3TB of data and that was quite a large part of why the recovery was longer than expected. Nevertheless, we managed to recover what the client had asked for and he and the company were pretty satisfied with the results.

​Get advice and assistance from Andy and the team at R3. R3 Data Recovery is real lab that deals with real disasters each an every day. If you have any sort of problem with a ​RAID or any data storage device, we are the people to contact. Call us today at 0800 999 3282 to get started with your unique case.