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iSCSI parity array

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I tried using Unraid as a datastore for a hypervisor but it's not good. Not good at all. So I flipped it, and made Unraid the hypervisor, and started storing data on it for once.


The server I converted only has drive bays for 2.5" drives though. A lot of them but only 2.5".

I only have flash drives on that format; they're faster and much cheaper — I checked just now: the most expensive flash drive at a local store is about USD35 sales tax included for a 1TB unit, which is USD70 for the same in magnetic — and according the the docu it's fine using them in the array. I'm not using a cache disk because I don't need it if all the disks are flash already, isn't that the point? That's unless parity is block based rather than file-based; then dual disks might be faster, but there's no indication of that, from what I gather. So that's that.


That leaves out parity. As I understand, parity is written in sync (practically and technically or programmatically; as in [mount] sync/async) with the array data, I'm using the fast mode on hybrid ZFS, split-any, most-free, BTW. Additionally, due to TRIM and star alignment or whatever, it can't be a flash drive, which limits my options to 7200RPM magnetic disks because I don't think they make large enough SAS drives or at least not without the enterprise price tag. For an all flash array I think that might be still too slow so I'm thinking, with the help of iSCSI moving it off server and use a RAID 0 or a RAID 10 array for the parity volume.


Is that allowed? Does Unraid need a disk or a volume? I don't remember its name but iSCSI has a command to unallocate/vacate or something to that effect, I see it on vSphere all the time. Is that enough for the parity volume?






Sidenote: this experiment made me think it's about time to flash stops being considered the exception, since it's really the norm. Maybe then we'd be more prepared for the sudden catastrophic failures of flash. It would seem either we missed out on the transition to flash, are in denial, or have not purchased a laptop in the last decade, huh? It's most ironic on ZFS from the BSD people, which, they make to distrust the disk with the idea of having no special requirements for its ultra resiliency, yet they have all of these special requirements on vdevs and last time I checked, flash too—though it was getting better.


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At the moment SSDs in the main Unraid style array cannot be trimmed, but they can in a ZFS pool which is what I think you are thinking of using.  You mentioned flash dries, though, did you mean that or SSD.   Also, how are these dries connected?   USB is subject to temporary drive disconnects which can cause problems in parity protected arrays.


The current release of Unraid DOES have a requirement of at least one drive in the main Unraid style array but if you do not intend to store anything on it then you can simply use a small USB flash drive to satisfy this requirement.   It is expected a future Unraid release will remove this restriction.


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  • 1 month later...

Thanks for answering and sorry for taking this long,


Earlier I meant permanently attached storage by the way, not removable storage. The server has an 8-port SAS/SATA controller. By the looks of it, this is only the first release of Unraid with ZFS support, I thought it already had support for a while because after starting using it again, it took a long time to discover it wasn't checking for notifying of updates, and it was very outdated. I just assumed support was already mature.


I had been drafting a question over what you mention, for weeks now; the required Unraid array in a pure ZFS setup (I wasn't sure if it was my mistake preventing the start of the pool), so I've been thinking about exploring the pure Btrfs pool angle. My question was about that. RAID5/6 aside, Btrfs is kinda low key superior to ZFS because of its flexibility, but there's nothing about Btrfs metadata/system/data profiles or setting up a pool outside of the context of cache.


Just today in the morning, I was still on it, making some drawings (attached) to help me illustrate a post I couldn't get under control and then…power went out. Talk about anticlimactic.


It's funny now. The silver lining though, is that I have much less to research now, thank you very very much, seriously.






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