Is it possible to reserve space on disks?

[wsw70]

I have disks of max size 8 TB - one of them is the parity disk for SnapRAID.

Since the parity disk must be of the size of the largest data disk and a little bit more I wanted to check if there is a way for MergerFS to reserve some space on all disks (as a percentage or absolute size), ensuring that data is not written there?

[trapexit]

What filesystem? ext4 has reserve storage. And you can use minfreespace on mergerfs to set a general boundry.

[wsw70]

What filesystem? ext4 has reserve storage. And you can use minfreespace on mergerfs to set a general boundry.

Thank you! This is exactly what I was looking for (I use ext4). I will go for minfreespace after reading about the pros and cons. Thanks!

[KutX9JtmXrZP]

I realize this has been answered, but wouldn't it just have been easier to create a partition smaller than the full disk size? So for an 8TB disk, just make a partition of 7TB?

[trapexit]

Creating multiple partitions could lead to worse performance if both are accessed as it guarantees that data is spread apart. I'm not sure if the kernel is smart enough to notice it's the same device and try to optimize the requests to the device. Would possibly want to check that. To be fair any random access to hdds is going to murder performance. But in the case of snapraid a sync will guarantee to cause both partitions to be actively used and that could lead to a serious perf decline.

[wsw70]

Creating multiple partitions could lead to worse performance if both are accessed as it guarantees that data is spread apart. I'm not sure if the kernel is smart enough to notice it's the same device and try to optimize the requests to the device. Would possibly want to check that. To be fair any random access to hdds is going to murder performance. But in the case of snapraid a sync will guarantee to cause both partitions to be actively used and that could lead to a serious perf decline.

This is certainly something to consider, but in my case the activity is not especially high and these are HDDs anyway.

[KutX9JtmXrZP]

Yes, that's one possible way to do it. But so are the others I mentioned and it doesn't require messing with partitions which are more of a pain to change.

Especially if they are encrypted

[KutX9JtmXrZP]

This is certainly something to consider, but in my case the activity is not especially high and these are HDDs anyway.

With mechanical drives there are many points of (mechanical) failure in addition to the electrical. For example you have the platter spinning (motor 1) but then you also have the r/w head on an arm that goes back and forth (motor 2). Having multiple data partitions on 1 drive will cause excessive "bouncing" of the r/w head at a minimum.

As trapexit said, given the array based nature of mergerfs, all you'll accomplish is a slowdown with increased wear and tear. Magnified by using snapraid.

If that second partition is not part of the mergerfs pool or snapraid pool - meaning you use it for something else entirely - go for it. For example things you want to backup but want to exclude from snapraid

But setting "minfreespace" in your mergerfs configs gives you maximum flexibility AND easiest maintenance

[wsw70]

But setting "minfreespace" in your mergerfs configs gives you maximum flexibility AND easiest maintenance

Yes, this is what I did.