SOLVED 4 Disks, Mirrored - Is This Right?

[dakotta]

I'm not sure I did this right.

I have only 4 disks.

I have read about the difference between using 2 mirrored vdevs vs RAID-Z2, and have decided that, for my use case, it's a toss-up. I decided to go with the mirror for the simplicity of rebuilding.

When I set up my pool, I selected all 4 drives and then clicked "mirror". For some reason I was expecting to set up a 2-disk striped vdev, then setup a second striped vdev and then tell TrueNAS to mirror the first to the second.

Did TrueNAS automatically do that when I clicked the "mirror" button? Or did I setup my pool incorrectly?

EDIT - or did I just create a 4-way mirror?

Cheers,

 

[Patrick M. Hausen]

zpool status or the UI will tell you ...

 

[sretalla]

When I set up my pool, I selected all 4 drives and then clicked "mirror". For some reason I was expecting to set up a 2-disk striped vdev, then setup a second striped vdev and then tell TrueNAS to mirror the first to the second.

Even if it had automatically done what you intended, you would have ended with a stripe of 2 mirrors, not a mirror of 2 stripes.

VDEVs are striped together in a pool, that's the only way.

As @Patrick M. Hausen said, zpool status -v will show you.

 

[dakotta]

Even if it had automatically done what you intended, you would have ended with a stripe of 2 mirrors, not a mirror of 2 stripes.

VDEVs are striped together in a pool, that's the only way.

As @Patrick M. Hausen said, zpool status -v will show you.

If I understand you correctly: it doesn't matter how many vdevs I have in my pool, data written to my pool is always striped across all of them. This is why a single-disk vdev is so dangerous for my data: losing that disk will destroy the vdev, which in turn will destroy the pool.

So, I don't actually have to tell TrueNAS what to do with the vdevs I create: it will always stripe data across all available vdevs.

What I have to tell TrueNAS is how many vdevs to create and how to structure the data in those vdevs.

In using the wizard, I selected 4 disks and told TrueNAS to create a mirrored pool. I'm pretty sure it created one vdev, in a 4-way mirror (see below). To achieve what I wanted to do, I should have selected two disks and created a mirror vdev. Then selected two more discs and created another mirror vdev. TrueNAS will then stripe data across those two vdevs... and any data written to a particular vdev will be mirrored across both discs in that vdev.

Introduction to ZFS
Cyberjock's slideshow

If I understand the report below, it shows a single vdev ( named mirror-0 ) with four disks in a 4-way mirror.

Code:

########## ZPool status report summary for all pools on server TRUENAS ##########

+--------------+--------+------+------+------+----+----+--------+------+-----+
|Pool Name     |Status  |Read  |Write |Cksum |Used|Frag|Scrub   |Scrub |Last |
|              |        |Errors|Errors|Errors|    |    |Repaired|Errors|Scrub|
|              |        |      |      |      |    |    |Bytes   |      |Age  |
+--------------+--------+------+------+------+----+----+--------+------+-----+
|boot-pool     |ONLINE  |     0|     0|     0|  2%|  0%|     N/A|   N/A|  N/A|
|tank          |ONLINE  |     0|     0|     0|  0%|  0%|     N/A|   N/A|  N/A|
+--------------+--------+------+------+------+----+----+--------+------+-----+

########## ZPool status report for boot-pool ##########

  pool: boot-pool
 state: ONLINE
config:

    NAME        STATE     READ WRITE CKSUM
    boot-pool   ONLINE       0     0     0
      da4p2     ONLINE       0     0     0

errors: No known data errors

########## ZPool status report for tank ##########

  pool: tank
 state: ONLINE
config:

    NAME                                            STATE     READ WRITE CKSUM
    tank                                            ONLINE       0     0     0
      mirror-0                                      ONLINE       0     0     0
        gptid/74d65684-f72b-11eb-a7fa-3417ebecd9d8  ONLINE       0     0     0
        gptid/77e8fb8b-f72b-11eb-a7fa-3417ebecd9d8  ONLINE       0     0     0
        gptid/78833559-f72b-11eb-a7fa-3417ebecd9d8  ONLINE       0     0     0
        gptid/7890cf3d-f72b-11eb-a7fa-3417ebecd9d8  ONLINE       0     0     0

errors: No known data errors

 

[Patrick M. Hausen]

Correct. But that can easily be fixed.

Code:

zpool detach tank gptid/78833559-f72b-11eb-a7fa-3417ebecd9d8
zpool detach tank gptid/7890cf3d-f72b-11eb-a7fa-3417ebecd9d8
zpool add tank mirror gptid/78833559-f72b-11eb-a7fa-3417ebecd9d8 gptid/7890cf3d-f72b-11eb-a7fa-3417ebecd9d8

 

[dakotta]

Oh. Very nice! Thank you. :)

 

[sretalla]

If I understand you correctly: it doesn't matter how many vdevs I have in my pool, data written to my pool is always striped across all of them. This is why a single-disk vdev is so dangerous for my data: losing that disk will destroy the vdev, which in turn will destroy the pool.

So, I don't actually have to tell TrueNAS what to do with the vdevs I create: it will always stripe data across all available vdevs.

What I have to tell TrueNAS is how many vdevs to create and how to structure the data in those vdevs.

That's a perfect summary of how it is.

Although I would add the slight caveat that the striping isn't always even across all of the VDEVs for a bunch of technical reasons including the timing of the VDEVs being added (at the beginning or later) and VDEV sizes/free space. In principle, for a pool that has always had the same number of equally-sized VDEVs, it's correct.

 

[dakotta]

That's a perfect summary of how it is.

Although I would add the slight caveat that the striping isn't always even across all of the VDEVs for a bunch of technical reasons including the timing of the VDEVs being added (at the beginning or later) and VDEV sizes/free space. In principle, for a pool that has always had the same number of equally-sized VDEVs, it's correct.

Sure.

I assumed it was more complicated than I was suggesting, but that was all my tiny, newbie-brain can handle right now.

Cheers,