I think the most effective approach in this case is to set the requirements and start from them.

This is not trivial advice :-) Requiring 4 drives in RAID10 has different consequences from 2 drivers in RAID1. Requiring a price < 100$ is different from < 200$. Hacked storage setup (eg. USB drives) versus proper (eg. storage controller with multiple SATA ports) is different.

For example, if you're willing to spend more, but not so much more, than say, 80$, why limiting to the RPi? The Odroid H2, for example, gives a lot of options.

Among the other things, I use it as file server with a COW-filesystem on two (cheap) SSD disks in RAID1. The board has two SATA ports, so it's a solid hardware setup. It has two ethernet ports, so it's ready to work as router. It's also x86, so it will have software support for a very long time¹. And it also runs at an incredibly cool temperature - I bought the fan, but it rarely spins. To be fair though, the H2 cases are expensive trash.

You can certainly do pretty much everything on a RPi4, as much as you can commute 50 km a day with a bike, which is... not ideal :-)

¹=RPi have an enthusiastic community, but ARM boards are second class citizens when it comes to Linux support. While Linux is formally compatible, it's still adviced to run the specific ARM distro.

You could set up a couple of GlusterFS or Ceph nodes. With the RPi4 having USB3 and Gigabit Ethernet you no longer get cramped by the previous generation's internal USB 2.0 Hub.

Usually you can get a new NAS really cheap on Black Friday. Even today you can get a 119j for $103, which isn't much more than a rpi with card and case.

https://www.newegg.com/synology-ds119j/p/N82E16822108418

A one-bay NAS... didn’t even know those existed

Yeah I use as backup, and it syncs to the cloud. So I figure if it dies I'm still good as I have the original and cloud. It also means I dont have to pay for 2 disks or think about RAID. It is slow and there is a learning curve but mostly just works and is very low power - at it shuts down disk when not in use.

> I have a file server running AMD Phenom II 955 that I'm thinking of replacing with a RPi. Is there a practical way to have RPis manage a software RAID?

I did this using an rpi2. It is non-trivial in terms of effort.

In a practical sense the rpi is limited in terms of power to the USB ports, so any build with 2+ HDDs and some sort of wireless USB keyboard and mouse dongle will use up too much power.

In other words, I had to buy a powered USB hub.

I used mdadm to do a software RAID, which more or less worked out of the box but took a while to clone the original drive. Throughput wasn't amazing for anything except occasional pulls and puts; I wasn't going to stream 4K video off of this, though I did have some luck with Subversion and music streaming.

It was clunky enough that I got a Synology box a year and a half later and never looked back.

It's a bit difficult since the RPi doesn't have any form of storage controller, just a USB hub. So your system would have to be built out of externally powered USB drives, which would be slow even by your lax standards and likely finicky.

It's hard to see the advantage over a dedicated NAS device, since the savings on the controller gets eaten up by needing to buy a whole bunch of drive enclosures and power supplies.

I'm (still) in the process of re-purposing an old laptop for a home server. 2TB laptop hard drives are a reasonable price, and on older laptops with a CD/DVD drive bay, you can usually get an adapter caddy to put in a second drive. Pay attention to the allowable drive height though.

This system is going to be set up with software RAID.

I expect to have higher compute power, more RAM, and much better IO throughput than a RPi solution. Also, the laptop battery (which is still good) serves as a built-in UPS.

> Is there a practical way to have RPis manage a software RAID?

I can't think of any good reason to do use software RAID, let alone doing so on a Raspberry Pi. I would recommend hardware RAID, ZFS, or a Synology appliance.

If performance is truly not a concern, yes it is possible to do this with a Pi but I don't think you are going to be super impressed.

Software RAID = simple. At least when I built this machine nearly 10 years ago. Since then it's been rock solid, if not blazing fast. There were also horror stories of hardware raids failing due to controllers failing; or subtle differences between different revisions of RAID controllers. That said I also had budgetary restrictiions at the time that made software raid attractive.

I looked at NAS appliances, but I think it's still cheaper to create one from scratch (assuming existing knowhow, and "free" time :)

ZFS is 100% software raid.

Sorta. At the end of the day, yes, the disks are being managed by your CPU and not a dedicated raid controller... but it's a completely different ballgame than something like mdadm.

Not "sorta", it's software raid and thus, like software raid, it's portable to any system that'll run the software and JBOD the drives.

Most would argue RAIDZ is better, but many of these reasons are functionality based over and about just redundancy, i.e. Copy on Write, et all.

Friends don't let friends create RaidZ pools.

Bite the bullet of the cost of N*2 mirrors.

I say sorta because in most circles when one is speaking of 'software RAID' they are referring to a very specific configuration and are explicitly not referring to ZFS.

ZFS doesn’t support hardware RAID controllers?

It supports it, but ZFS itself is operating on the host CPU, not the RAID controller. Additionally, ZFS prefers that the RAID controller is disabled/defeated and operating in JBOD mode where disks are passed-thru to the OS without any caching.

Not just caching. The problem is that non-JBOD HBAs also swallow error codes and mask retries, meaning ZFS is potentially unaware of some read errors and is not in control of the latency as the controller firmware has its own timeouts and retry counters. Moreover, most will not pass through SMART or else fake it.

Not a Pi but the rockpro64 has a pcie port in which you can plug what you want (a sata card for example).

I made a nas with one 3tb drive and zfs. Zfs was a bit difficult to install but now it works really well and the power consumption is low.

You have to use the Linux images mades by ayufan (a third party) though. The manufacturer just list a bunch of distro that run on the cards and most of them seem to be made by some volunteer. I'm not a big fan of that.

There is nothing wrong with software raid. Synology uses SW raid in all their NAS products as far as I know, right up to the flagship models with > 10 drives and 100T+ of storage.

Hardware raid controllers are a single point of failure and when they fail the vendor sells a different product so you will always have to stock up and buy two at once.

Software RAID is easier to manage, easier to do disaster recovery, does not require specific driver support, and easier to move to other hardware.

maybe the Pi 4 would be able to do this with a USB attached disk box, but you'd be much better off with something like a commercial NAS - and I say this as someone who has used ARM boards since the Sheevaplug.

It all depends - do you want a fileserver or a crash course in how to build a fileserver?

Maybe if you could use something like AoE turning the network port into the backend storage interface a la CoRaid. Otherwise like others have said you have USB.

There are one or two NanoPi models with an M.2 slot and full gigE, but that doesn’t get you an array unless there’s some m.2 breakout box I’m not familiar with.

For RAID, no. The pi has a few strikes against it for that case. If you're simply after redundancy, ZFS may work for your case.