I know how RAID work and prevent data lost from disks failures. I want to know is possible way/how easy to recover data from unfunctioned remaining RAID disks due to RAID controller failure or whole system failure. Can I even simply attach one of the RAID 1 disk to the desktop system and read as simple as USB disk? I know getting data from the other RAID types won’t be that simple but is there a way without building the whole RAID system again. Thanks.
RAID is more likely to fail than a single disk. You have the chance of single-disk failure, multiplied by the number of disks, plus the chance of controller failure.
RAID 1 and RAID 5 protect against that by sharing data across multiple disks, so you can re-create a failed drive, but failure of the controller may be unrecoverable, depending on availability of new, exact-same controller. With failure of 1 disk in RAID 1, you should be able to use the array ‘degraded,’ as long as your controller still works. Depending on how the controller works, that disk may or may not be recognizable to another system without the controller.
RAID 1 disks are not just 2 copies of normal disks. Example: I use software RAID 1, and if I take one of the drives to another system, that system recognizes it as a RAID disk and creates a single-disk, degraded RAID array with it. I can mount the array, but if I try to mount the single disk directly, I get filesystem errors.
I’ve never been a big fan of RAID for this reason… but I’ve also never had enough mission critical data that I couldn’t just store hard copy backups.
That being said… let me ask you this:
Is there a better way than RAID for data preservation/redundancy?
Just for drive redundancy it’s awesome. One drive fails you just pull it out, put in a new one and let the array rebuild. I guess the upside of hardware RAID is that some even allow you to swap a disk without powering down. Either way, you have minimal downtime.
I guess a better way would be to have multiple servers. Though with features like checksums in BTRFS I guess a RAID is still better because it can protect against bitrot. And with directly connected systems in a RAID it is generally easier to ensure consistency.
Yeah, that’s generally my consensus as well. Just curious if someone had a better way that maybe I didn’t know about.
Are there differences in the context of failure, when using a controller vs software raid with mdadm?
With software raid, there is no controller to fail.
Well, that’s not strictly true, because you still have a SATA/SAS controller, HBA, backplane, or whatever, but they’re more easily replaceable. (Unless it’s integrated in the motherboard, but then it’s not a separate component to fail.)
RAID is more likely to fail than a single disk. You have the chance of single-disk failure, multiplied by the number of disks, plus the chance of controller failure.
This is poorly phrased. A raid with a bad disk is not failed, it is degraded. The entire array is not more likely to fail than a single disk.
Yes, you are more likely to experience a disk failure, but like you said, only because you have more disks in the first place. (However, there is also the phenomenon where, after replacing a failed disk, the additional load during the rebuild might cause a second disk to fail, which is why you should replace failed disks as soon as possible. And have backups.)
recover data from unfunctioned remaining RAID disks due to RAID controller failure
In this case, you need a new RAID controller of similar type.
Can I even simply attach one of the RAID 1 disk to the desktop system
No. One disk out of a RAID array is different from a normal disk.
Recovery becomes easy if you do not use a hardware RAID controller, but a ZFS software RAID instead. It does nearly all automatically. But you need to do a little more reading tutorials for the first setup.
Lots of people have moved away from raid entirely because of some of these issues. There are alternatives these days. For example mergerfs or the ZFS file system.
Can confirm that moving a zfs array to a new system after a failure is simply connect the disks and zpool import -f <pool_name>
Every raid card I use now is put in hba mode it’s just simpler to deal with
Not to mention hardware raid is dependent on a functional card. You are in trouble if the card has problems. Also hardware raid doesn’t do data validation
You really do not want hardware raid. You want software raid like LVM or better yet, ZFS.
Do your own research. Keep in mind raid isn’t a backup. It is only for convenience.
In my experience, using spinning disks, the performance is very poor, and times for scrub and resilver are very long. For example, in a raidz1 with 4x8TB, scrubbing takes 2-3 weeks and resilvering takes almost 2 months. I must also add very poor performance in degraded state. This is a very old post, but things are still the same.
A lot of “hardware raid” is just a separate controller doing software raid. I thought I lost access to a bunch of data when my raid controller died, before I realized that I could just plug the disks directly into the computer and mount them with mdadm. But yes, hardware raid seems a bit pointless nowadays.
For recovering hardware RAID: most guaranteed success is going to be a compatible controller with a similar enough firmware version. You might be able to find software that can stitch images back together, but that’s a long shot and requires a ton of disk space (which you might not have if it’s your biggest server)
I’ve used dozens of LSI-based RAID controllers in Dell servers (of both PERC and LSI name brand) for both work and homelab, and they usually recover the old array to the new controller pretty well, and also generally have a much lower failure rate than the drives themselves (I find myself replacing the cache battery more often than the controller itself)
Only twice out of the handful of times I went to a RAID controller from a different generation
- first time from a mobi failed R815 (PERC H700) physically moving the disks to an R820 (PERC H710, might’ve been an H710P) and they were able to foreign import easily
- Second time on homelab I went from an H710 mini mono to an H730P full size in the same chassis (don’t do that, it was a bad idea), but aside from iDRAC being very pissed off, the card ran for years with the same RAID-1 array imported.
As others have pointed out, this is where backups come into play. If you have to replace the server with one from a different generation, you run the risk that the drives won’t import. At that point, you’d have to sanitize the super block of the array and re-initialize it as a new array, then restore from backup. Now, the array might be just fine and you never notice a difference (like my users that had to replace a failed R815 with an 820), but the result pattern is really to the extremes of work or fault with no in between.
Standalone RAID controllers are usually pretty resilient and fail less often than disks, but they are very much NOT infallible as you are correct to assess. The advantage to software systems like mdadm, ZFS, and Ceph is that it removed the precise hardware compatibility requirements, but by no means does it remove the software compatible requirements - you’ll still have to do your research and make sure the new version is compatible with the old format, or make sure it’s the same version.
All that’s said, I don’t trust embedded motherboard RAIDs to the same degree that I trust standalone controllers. A friend of mine about 8-10 years ago ran a RAID-0 on a laptop that got it’s super block borked when we tried to firmware update the SSDs - stopped detecting the array at all. We did manage to recover data, but it needed multiple times the raw amount of storage to do so.
- we made byte images of both disks in ddrescue to a server that had enough spare disk space
- found a software package that could stitch together images with broken super blocks if we knew the order the disks were in (we did), which wrote a new byte images back to the server
- copied the result again and turned it into a KVM VM to network attach and copy the data off (we could have loop mounted the disk to an SMB share and been done, but it was more fun and rewarding to boot the recovered OS afterwards as kind of a TAKE THAT LENOVO…we were younger)
- took in total a bit over 3TB to recover the 2x500GB disks to a usable state - and took about a week of combined machine and human time to engineer and cook, during which my friend opted to rebuild his laptop clean after we had images captured - to one disk windows, one disk Linux, not RAID-0 this time :P
Why wouldn’t you just use software raid? It is way more robust and if you are using ZFS you get all the nice features that come as a part of it.
I never said I didn’t use software RAID, I just wanted to add information about hardware RAID controllers. Maybe I’m blind, but I’ve never seen a good implementation of software RAID for the EFI partition or boot sector. During boot, most systems I’ve seen will try to always access one partition directly and a second in order, which is bypassing the concept of a RAID, so the two would need to be kept manually in sync during updates.
Because of that, there’s one notable place where I won’t - I always use hardware RAID for at minimum the boot disk because Dell firmware natively understands everything about it from a detect/boot/replace perspective. Or doesn’t see anything at all in a good way. All four of my primary servers have a boot disk on either a Startech RAID card similar to a Dell BOSS or have an array to boot off of directly on the PERC. It’s only enough space to store the core OS.
Other than that, at home all my other physical devices are hypervisors (VMware ESXi for now until I can plot a migration), dedicated appliance devices (Synology DSM uses mdadm), or don’t have a redundant disks (my firewall - backed up to git, and my NUC Proxmox box, both firewalls and the PVE are all running ZFS for features).
Three of my four ESXi servers run vSAN, which is like Ceph and replaces RAID. Like Ceph and ZFS, it requires using an HBA or passthrough disks for full performance. The last one is my standalone server. Notably, ESXi does not support any software RAID natively that isn’t vSAN, so both of the standalone server’s arrays are hardware RAID.
When it comes time to replace that Synology it’s going to be on TrueNAS
Your information is about 10 years out of date. It is trivial to do boot with raid as in EFI you just set both drives as bootable.
I think hardware raid is only for the last resort as Windows has Storage sense and Linux has ZFS, LVM and mdadm. I’ve never heard of a hardware raid system that has the features a lot of these systems have like data integrity checking and ram caching.
Essentially I don’t really see a need for hardware raid in a home environment and there isn’t a huge need in the business.
I can confirm that moving the disks to a very similar device will work.
We recovered “enough” data from what disks remained of a Dell server that was dropped (PSU side down) from a crane. The server was destroyed, most of the disks had moved further inside the disk caddy which protected them a little more.
It was fun to struggle with that one for ~1 week
And the noise from the drives…
