md takes multiple partitions to make a virtual device you can put a file system on, with striping and traditional RAID levels
unRaid takes multiple partitions, dedicates one or two of them to parity, and hands the other partitions through. You can handle those normally, putting different file systems on different partitions in the same array and treating them as completely separate file systems that happen to be protected by the same parity drives
This enables you to easily mix drives of different sizes (as long as the parity drives are at least as large as the largest data partition), add, remove or upgrade drives with relative ease, and means that every read operation only goes to one drive, and writes to that drive plus the parity drives. Depending on how you organize your files you can have drives that are basically never on, while in an md array every drive is used for basically every read or write.
The disadvantages are that you lose out on the performance advantages of a RAID, and that the raid only really protects against losing entire disks. You can't easily repair single blocks the way a zfs RAID could. Also you have a number of file systems you have to balance (which unRaid helps you with, but I don't know how much of that is in this module)
Not sure what you mean by 'easily repair single blocks the way a zfs RAID could', but often the physical devices handle bad blocks, and md has one safety layer on top of this - bad blocks tracking. No relocation in md though, AFAIK.
Ignoring its biggest advantage vs mdraid, or zfs raid:
The ability to sleep all / individual HDDs:
* only keep awake the drives that your actually read data from
* only keep awake the drive that your write data too + n parity drives
For home users, that is a TON of energy saving. And no, your "poor" HDDs are not going to suffer from spinning up a few times per day.
You can spin up/down a HDD 10x per day, for 100 years before you come even close to the manufactures (lowest) hdd limits. Let alone if you have 4+ drives and have a bit of data spreading, or combined with unraids nvme/ssd caching layer.
So unlike mdraid or zfs where its a all or nothing situation, unraid / snapraid gives you a ton of energy saving.
And i understand the US folks here do not care when they pay maybe 6 to 12 cent / kwh, but the rest of the world has electricity prices in the 30 to 50 cent / kwh, and it stacks up very fast when you are using < 1watt vs 5/7Watt per HDD/24/7...
I have an unRAID homelab. It's kind of really awesome in the sense that it lets the home user incrementally add compute and capacity over time without having to do it all in one big shebang and doesn't require nearly the fussing over of my prior linux server.
I started mine with a spare NUC and some portable USB drives and its grown into a beast with over 100TB spread across a high performance SSD backed ZFS pool and an unRAID array, 24 cores, running about 20 containers and a few VMs without breaking a sweat and so far (knock on wood) zero data loss.
All at a couple hundred dollars every so often over the years.
One performance trick it supports is also letting you overlay fast SSD storage over the array, which is periodically moved onto the slower underlying disk. It's transparent, so when you write to the array you can easily get several hundred MB/sec which will automatically get moved onto warm storage periodically. I have two fast SSDs RAIDed there and easily saturate the network link when writing.
The server basically maintains itself, I only go in every so often and bump the docker containers at this point. But I also know that I can add another disk to it in a about 10 minutes and a couple hundred bucks.
> If a drive fails, you rebuild it from the parity.
But if the file system is corrupt then you're hosed and end up with a `lost+found`. It sounds great until it fails, and then you realize why ZFS with replication makes sense. Unraid doesn't do automatic repairs from replicated ZFS datasets yet either even if you use individual ZFS disks within your Unraid array.
I have an issue with this though... Won't you get a write on the parity drive for each write on any other drive? Doesn't seem well balanced... to be frank, looks like a good way to shoot yourself in the foot. Have a parity drive fail, then have another drive fail during the rebuild (a taxing process) and congrats -- your data is now eaten, but at least you saved a few hundred dollars by not buying drives of equal size...
wongarsu|7 months ago
unRaid takes multiple partitions, dedicates one or two of them to parity, and hands the other partitions through. You can handle those normally, putting different file systems on different partitions in the same array and treating them as completely separate file systems that happen to be protected by the same parity drives
This enables you to easily mix drives of different sizes (as long as the parity drives are at least as large as the largest data partition), add, remove or upgrade drives with relative ease, and means that every read operation only goes to one drive, and writes to that drive plus the parity drives. Depending on how you organize your files you can have drives that are basically never on, while in an md array every drive is used for basically every read or write.
The disadvantages are that you lose out on the performance advantages of a RAID, and that the raid only really protects against losing entire disks. You can't easily repair single blocks the way a zfs RAID could. Also you have a number of file systems you have to balance (which unRaid helps you with, but I don't know how much of that is in this module)
phoronixrly|7 months ago
eddythompson80|7 months ago
- It lets you throw JBODs (of ANY size) and you can create a "RAID" over them.
- The biggest drive must be a parity drive(s).
- N parity = surviving N drive failures.
- You can expand your storage pool 1 drive at a time. You need to recalculate parity for the full array.
The actual data is spread across drives. If a drive fails, you rebuild it from the parity. This is another implementation (using MergerFS + SnapRAID) https://perfectmediaserver.com/02-tech-stack/snapraid/
It's a very simple model to think of compared to something like ZFS. You can add/remove capacity AND protection as you go.
Its perf is significantly less than ZFS of course.
benjiro|7 months ago
The ability to sleep all / individual HDDs:
* only keep awake the drives that your actually read data from * only keep awake the drive that your write data too + n parity drives
For home users, that is a TON of energy saving. And no, your "poor" HDDs are not going to suffer from spinning up a few times per day.
You can spin up/down a HDD 10x per day, for 100 years before you come even close to the manufactures (lowest) hdd limits. Let alone if you have 4+ drives and have a bit of data spreading, or combined with unraids nvme/ssd caching layer.
So unlike mdraid or zfs where its a all or nothing situation, unraid / snapraid gives you a ton of energy saving.
And i understand the US folks here do not care when they pay maybe 6 to 12 cent / kwh, but the rest of the world has electricity prices in the 30 to 50 cent / kwh, and it stacks up very fast when you are using < 1watt vs 5/7Watt per HDD/24/7...
bane|7 months ago
I started mine with a spare NUC and some portable USB drives and its grown into a beast with over 100TB spread across a high performance SSD backed ZFS pool and an unRAID array, 24 cores, running about 20 containers and a few VMs without breaking a sweat and so far (knock on wood) zero data loss.
All at a couple hundred dollars every so often over the years.
One performance trick it supports is also letting you overlay fast SSD storage over the array, which is periodically moved onto the slower underlying disk. It's transparent, so when you write to the array you can easily get several hundred MB/sec which will automatically get moved onto warm storage periodically. I have two fast SSDs RAIDed there and easily saturate the network link when writing.
The server basically maintains itself, I only go in every so often and bump the docker containers at this point. But I also know that I can add another disk to it in a about 10 minutes and a couple hundred bucks.
somat|7 months ago
xcrunner529|7 months ago
hammyhavoc|7 months ago
But if the file system is corrupt then you're hosed and end up with a `lost+found`. It sounds great until it fails, and then you realize why ZFS with replication makes sense. Unraid doesn't do automatic repairs from replicated ZFS datasets yet either even if you use individual ZFS disks within your Unraid array.
phoronixrly|7 months ago