Pool RAID questions

[EArroyo]

I recently discovered my cache pool was set to RAID1 and reason why I was not seeing double the space I was expecting.  I have 2 1TB SSDs on my pool.

I have 2 questions:

 

1. Is changing the POOL RAID level destructive?  Will  I lose the data on the cache drives in the conversion?  Let's say from RAID1 (default) to RAID0 or Single?

 

2. If I chose RAID0 will I see any improvements since the bottleneck is when writing back and doing parity writes what's slowing things down?  So what would be the best option?  Single?  Or RAID0?   Network is Gigabit, so 120Mb is top transfer speed to my array from any other device/machine.

 

I hope I was clear enough in my questions.  If RAID0 has no real advantage then I would go Single just to duplicate my space.  Or am I confused in the "Single" option?

Edited September 9, 2020 by EArroyo

[trurl]

I have split your post into its own thread. Please don't post questions in the FAQ thread.

[JorgeB]

7 minutes ago, EArroyo said:

1. Is changing the POOL RAID level destructive?

No.

 

8 minutes ago, EArroyo said:

2. If I chose RAID0 will I see any improvements since the bottleneck is when writing back and doing parity writes what's slowing things down? 

Don't understand the question, cache has nothing to do with array parity, but raid0 should be faster, though if using gigabit network might be the bottleneck even with a single device.

[EArroyo]

So using it internally (since my max transfer speed via network will always be up to 120mbit) RAID0 will be faster than Single?  Has anybody been able to test this and confirm?   Because when writing you still have to go through the parity calculations, so my question is, will there be any real gains using a RAID0 over Single pool when you have bottlenecks (gigabit on network and parity on local array)

 

Sorry if you don't understand my question, I don't know how to put it simpler..  Just looking for people who have tested these scenarios and can confirm RAID0 will, for example, copy from cache to array faster or will the parity check/write still be the bottleneck and no speed gains will be seen.

[trurl]

23 minutes ago, EArroyo said:

RAID0 will, for example, copy from cache to array faster or will the parity check/write still be the bottleneck and no speed gains will be seen.

This question makes me wonder if you understand how cache works on Unraid. When writing to a cache-yes user share, parity is not involved. The write will happen at the speed of cache. Later, at the scheduled mover time, those files get moved to the parity array. At that point, parity is involved, but that is independent and long after the write to cache.

 

Parity is realtime, cache is not. Mover is intended for idle time. The default mover schedule is daily in the middle of the night.

[ChatNoir]

The real question is what kind of speed are you expecting ?

 

I can easily saturate my Gbs connection from my game PC to a RAID1 SSD pool.

And I generally copy at ~400MBs between two RAID1 pool (SATA SSDs).

 

In any case, parity or no, with the array you won't much higher than 2xx MBs. Unless you need very high speed for VMs, dockers and if you don't have a multigig network, RAID1 might be enough with the added safety of the second drive.

[Vr2Io]

8 hours ago, EArroyo said:

copy from cache to array faster or will the parity check/write still be the bottleneck and no speed gains will be seen.

Yes, no gain, no matter how fast of the cache pool.

 

This is another topic during data move to array and new data input to cache pool in same time.

Edited September 10, 2020 by Benson

[SeeGee]

If you're capped at Gigabit and only worried about network file storage, your cache and array disks only need to be capable of 120mb+ with regards to anything coming in or out of the network. 

In this case, raid0 offers no benefit. Not even when running mover, because it can only move files at whatever speed the destination drive can handle. Raid1 offers redundancy. (Safe Bet) 

 

Now, when you start including docker containers and virtual machine performance when the data for those features are also stored on the cache, then the speed benefit of raid0 starts to become more appealing. But no safety net if a disk fails. 

 

Some people will use an unassigned SSD to run docker/vms and have the best of both worlds

 

Edited September 11, 2020 by SeeGee

[EArroyo]

Perfect!   Thank you all!   Yes, I do understand how cache works, and yes, your answer is correct.  I meant MOVER.   So my question is answered.

 

So RAID0 vs Single is really a matter of choice rather than benefit.  I guess that arises a new question, will running my VM images off of the cache SSD (now a BTRFS pool) have benefit doing RAID0 over Single?  Again, seeking real-life throughput speeds.  My understanding is the controller is more than likely the bottleneck?

 

BTW, I used to work for DPT (now Adaptec) as the Performance Lab Engineer and used to run benchmarks on fibre channel arrays of 80+ drives (sounded like SUPER LOUD rain storm 🤣) and I am used to seeing crazy throughput numbers, believe me, but in real-life unRAID I know nothing about speeds.