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How RAID system reliable? possible of raid system failure

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.

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  • 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

    • 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...

    • 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 never said anything about EFI not supporting multi boot. I said that the had to be kept in lockstep during updates. I recognize the term "manual" might have been a bit of a misnomer there, since I included systems where the admin has to take action to enable replication. ESXi (my main hardware OS for now) doesn't even have software RAID for single-server datastores (only vSAN). Windows and Linux both can do it, but its a non-default manual process of splicing the drives together with no apparent automatic replacement mechanism - full manual admin intervention. With a hardware RAID, you just have to plop the new disk in and it splices the drive back into the array automatically (if the drive matches)
            • Dell and HPe both have had RAM caching for reads and writes since at least 2011. That's why the controllers have batteries :)
              • also, I said it only had to handle the boot disk. Plus you're ignoring the fact that all modern filesystems will do page caching in the background regardless of the presence of hardware cache. That's not unique to ZFS, Windows and Linux both do it.
            • mdadm and hardware RAID offer the same level of block consistency validation to my current understanding- you'd need filesystem-level checksumming no matter what, and as both mdadm and hardware RAID are both filesystem agnostic, they will almost equally support the same filesystem-level features (Synology implements BTRFS on top of mdadm - I saw a small note somewhere that they had their implementation request block rebuild from mdadm if btrfs detected issues, but I have been unable to verify this claim so I do not consider it (yet) as part of my hardware vs md comparison)

            Hardware RAID just works, and for many, that's good enough. In more advanced systems, all its got to handle is a boot partition, and if you're doing your job as a sysadmin there's zero important data in there that can't be easily rebuilt or restored.

49 comments