ZFS storage pdf theory question

[LemonBag]

Hello all,

New user here. I'm reading the manual provided by the "ZFS Storage Pool Layout" PDF and am having trouble understanding this section, page 7, on the PDF. I do not understand where the "Read IOPS: 1200" is being derived from.

Each drive in the example presented in the PDF is listed as "Read IOPS: 250". As I currently understand it, in a mirrored configuration, the read speeds are doubled since that work process can be split amongst the 2 copied drives. This leads to a total "Read IOPS" of 500. However, as the example goes on to extend into a "6 x 2-way mirror" metaphor, the math on my end should work out to either 3000 as a 1x2 array has 500 IOPS (500 x 6 = 3000) OR 1500 as each individual drive has the capability of 250 IOPS.

Where am I going wrong here? How did they come up with the stated 1200 Read IOPS 600 Write IOPS.

 

[HoneyBadger]

Hey @LemonBag

Can you tell us what URL you picked up the whitepaper from? We did catch this math error early on this year and published an updated copy:

https://www.truenas.com/solution-guides/#TrueNAS-PDF-zfs-storage-pool-layout/1/

 

[LemonBag]

ZFS Storage Pool Layout

This amazing document, created by iXsystems in February 2022 as a "White Paper", cleanly explains how to qualify pool performance touching briefly on how ZFS stores data and presents the advantages, performance and disadvantages of each pool...

www.truenas.com

 

[LemonBag]

Hey @LemonBag

Can you tell us what URL you picked up the whitepaper from? We did catch this math error early on this year and published an updated copy:

https://www.truenas.com/solution-guides/#TrueNAS-PDF-zfs-storage-pool-layout/1/

Yes, it is the link I posted to in a separate reply above. I don't see where the updated copy is. The screenshot in this post is sourced from the PDF obtained after clicking the orange download button in the top right corner.

 

[Davvo]

New user here. I'm reading the manual provided by the "ZFS Storage Pool Layout" PDF and am having trouble understanding this section, page 7, on the PDF. I do not understand where the "Read IOPS: 1200" is being derived from.

If you look at the description, before downloading the file, there is a button named "Show: ERRATA" that contains the following.

While the resource is valid there are a few mistakes, namely:

• page 4, Storage space efficiency formula
  • correct: 1/N​
• page 5, 6x 2-way mirror
  • correct Read IOPS: 3000​
  • correct Write IOPS: 1500​
• page 6, 4x 3-way mirror
  • correct Streaming read speed: 1200 MB/s​
• page 11, Scenario 3​
  • a SLOG is not a write cache​

@HoneyBadger I will update the resource with the corrected PDF later.

 

[LemonBag]

Ah yes I see this. Thank you

 

[Davvo]

@HoneyBadger I take the chance to report that the now page 5, 4x 3-way mirror streaming speed is still wrong (3000 MB/s instead of 1200 MB/s): from my understanding it should be 300 MB/s per 3-way mirror (single-drive reasind speed: 100 MB/s) multipled by 4 = 1200 MB/s; also, the SLOG is still adressed as a write cache in now page 10.

If you could provide me with a fixed document or make me understand why I'm wrong I will update the resource accordingly.

 

[HoneyBadger]

@Davvo we're updating the PDF again - there are some mathematical missives still, and I'll try to make the SLOG description a little more detailed without going too far down the rabbit hole and necessitating a whole separate document on it (yet)

 

[Arwen]

Perhaps the Resource should simply reference the truenas.com URL for the PDF. That way when iX makes an updated PDF, it is automatically available via the Resource page.

 

[Davvo]

I prefer to load the file for a bunch of reasons, backup being one of those.