Adding drives, affect on performance?

[egtrev]

I've been using unRAID for a while now in a 1 parity / 1 disk setup. I also have a btrfs RAID 1 cache but this is only used for appdata/VM's.

Usage is mainly just general data and photos being accessed/edited, nothing major. I max out my 1GbE network with 115Mbps transfers.

When I run out of space I just upgrade both drives, unfortunately 12TB/14TB currently seem to not be at a good value, so I'm looking at just throwing another 8TB in there.

I know this has been explained previously but after searching I can't seem to find the answer.

How much of a performance hit will I see when adding additional drives?

 

[JorgeB]

If you only have 1 data drive parity is basically working as a mirror (raid1), so it will be slower with 2 or more data disks, though you can maintain similar performance with turbo write, at the expense of all disks needing to be spun up for writes.

 

 

 

 

[trurl]

1 parity 1 data is a special case which results in a mirror. I think that might perform somewhat better with parity updates than the more general case of multiple data disks but I haven't tried it. Once you get more than 1 data disk it doesn't matter how many you have it works the same assuming no controller bottlenecks, since the disks are accessed in parallel. See this for more information on the 2 methods of configuring parity updates and their tradeoffs:

 

 

[egtrev]

Thanks for the info.

Turbo write aside, what kind of performance drops are expected if I go to 2 data drives.

Not looking for specific figures, but would it be say 50% slower?

Is this across write and read operations?

Also if I'm understanding correctly anything more than 2 data drives isn't going to affect performance compared to 2 data drives, so 3,4,5 data drives will perform similar to 2?

 

[JorgeB]

1 minute ago, egtrev said:

Not looking for specific figures, but would it be say 50% slower?

About that, expect write speeds around 40/60MB/s depending on disks used.

 

2 minutes ago, egtrev said:

Is this across write and read operations?

Only writes, read performance remains unchanged.

 

2 minutes ago, egtrev said:

Also if I'm understanding correctly anything more than 2 data drives isn't going to affect performance compared to 2 data drives, so 3,4,5 data drives will perform similar to 2?

Yep.

[egtrev]

Thanks Johnnie.

If I wanted to try this, is it as simple as just adding a new precleared drive to the array and testing it out?

Then if I'm not happy, clearing the new drive again and removing it from the array?

 

[JorgeB]

1 minute ago, egtrev said:

Then if I'm not happy, clearing the new drive again and removing it from the array?

Yes, but this is a manual process, alternatively you can do a new config and re-sync parity without it.