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I have an intel server of which the RAID Controller does not support JBOD mode (same problem as my Dell R820s).

But I want to do Ceph, so I "need" a JBOD. So I set my 3 SSDs as single drive RAID 1 from the controller configuration UI (maybe I should have done RAID 0 though).

Is there some performance or other drawback to this VS creating a real JBOD on a controller that provides the option? Is there going to be some RAID overhead that I'm not aware of?

I was also a little surprised that the controller let me create 3 separate RAID 1s from only 3 hard drives but it fits my use case so, hey, nice.

Johnride
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  • What is the most important? Fault-tolerance or performance. RAID 1 is used to reduce downtime in case a drive(s) dies whereas RAID 0 is to increase performance and JBOD is just to use the storage space of several drives as one. You need to have an understanding of these concepts and what best fits what you are trying to do before you proceed with any configuration. – Nasir Riley Nov 02 '21 at 06:11
  • I understand those concepts very well, those drives will be part of a Ceph cluster for fault tolerance. I only need to expose them individually to the host OS and configuring them as JBOD in the RAID controller is usually the way to "bypass" the raid controller as I don't need the controller to manage fault tolerance. – Johnride Nov 02 '21 at 11:24
  • Then what is the question exactly? If you actually did have an understanding, then you'd know that JBOD and RAID 0 aren't viable for what you are trying to achieve because they don't provide fault-tolerance to avoid downtime. RAID 0 is to increase performance in the case of content creation, for example, and JBOD is out of the question because one disk dying would destroy everything. I don't know exactly what you did, but one can't set up RAID 1 with three drives unless the third is a hot spare. The best option for reducing downtime for you is RAID 5 as you'd still be up with one disk dying. – Nasir Riley Nov 02 '21 at 12:03
  • Fault tolerance is handled by [Ceph](https://ceph.io/en/). The question is : Is there some performance or other drawback to this (single drive RAID 1, which I know does not provide fault tolerance) VS creating a real JBOD on a controller that provides the option? Is there going to be some RAID overhead that I'm not aware of? – Johnride Nov 02 '21 at 12:07
  • You cannot and do not have a single drive RAID 1 and even if you did, it would be useless because the protection against a drive dying wouldn't exist but since you don't need a RAID level to reduce downtime, it doesn't matter anyway. JBOD isn't RAID so no, there won't be any overhead. The only way that there would be overhead is if you were using RAID 5 or 6 because the parity bit would cause said overhead. – Nasir Riley Nov 02 '21 at 12:13
  • I know single drive RAID1 doesn't make sense and that's why I'm asking. RAID0 doesn't make much more sense in my setup either though. The parity bit is exactly the kind of overhead I'm asking about and it looks like RAID1 doesn't have it. My problem is, in other words : I need JBOD, my controller doesn't allow it, so I did single drive RAID 1s. What overhead can I expect vs a proper JBOD? – Johnride Nov 02 '21 at 16:05
  • You don't have a single drive RAID 1 as that isn't possible nor do you need the controller to set up JBOD. You can do the latter from within the operating system. If it's Linux or another Unix-based OS, then just create physical volumes using the three disks and the create a volume group and a logical volume. You can then add more disks to it later if you need it. You can set up what's basically the same thing in disk management in Windows. A Google search will show you how to do this. For what it's worth, I don't recommend JBOD because if one disk dies, you lose all of you data. – Nasir Riley Nov 02 '21 at 21:52
  • Read on Ceph, maybe you'll understand what I'm talking about. I don't want a large drive, I don't want RAID, I want Ceph which emulates on his own large drives from a distributed storage environment and manages redundancy on his own across multiple machines. It is explicitly not recommended to do RAID with Ceph as it replaces RAID altogether. See [ceph replace RAID](https://www.techrepublic.com/article/why-ceph-could-be-the-raid-replacement-the-enterprise-needs/) and [Ceph Architecture](https://docs.ceph.com/en/latest/architecture/) especially the data striping part. – Johnride Nov 03 '21 at 15:59
  • I have already addressed you not wanting RAID am not suggesting that you use it. Tour question and comments are regarding JBOD and how the controller doesn't support it. I've already covered this. – Nasir Riley Nov 03 '21 at 22:10

1 Answers1

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Is there some performance or other drawback to this VS creating a real JBOD on a controller that provides the option?

Likely not, but that depends on the actual controller at hand. RAID1 with just a single disk implies that there's a missing disk, so RAID0 would be more fitting by logic alone.

You should be aware that you're running that controller outside its normal operational envelope - somewhat unlikely but theoretically possible there could be performance issues but also logic problems like overspilling event logs. In any case, test well.

Preferably, look for a controller that explicit supports what you're doing and possibly consider a simple non-RAID SAS or SATA HBA.

Zac67
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  • Thanks that makes sense, I'll take the time to switch to RAID 0 as I was thinking. I'll be benchmarking. I also want to setup the host OS hard drives as RAID 1 on the same controller so my next option would be a controller that supports RAID 1 and JBOD. – Johnride Nov 04 '21 at 14:32