PSU Wattage and SATA Connection

[Davvo]

SLOG is only needed when you have lots of syncwrites (sync = always).
Only SLOG needs PLP.
Metadata VDEV is generally not needed in a flash pool (it's usually a way to increase an HDD pool performance).

 

[joeschmuck]

While I do have an all NVMe system, I do not use an SLOG or L2ARC, I just don't need them. Same with the Metadata VDEV.
I'd recommend that you build the system without all the fancy add-ons and just make it work. You can always add an SLOG later if you find you may need it, but spend the money for the power loss prevention or forget about using an SLOG.

But in the old days they had to pour a pad and put in a new transformer to boot the HDD in the Vax 750 / PDP 11 at school. That thing was a beast

I remember those things, had them at work (government program) in 1990's and I know they had been there for many years. I don't recall if it was a 750 model but we did have the RA60 disk drive. I actually went through our cabinets at work just before Christmas and purged a few shelves of old DEC manuals and schematics for the computers and drives. I thought about holding on to some as a novelty but I have enough crap at home already.

 

[Fastline]

While I do have an all NVMe system, I do not use an SLOG or L2ARC, I just don't need them. Same with the Metadata VDEV.
I'd recommend that you build the system without all the fancy add-ons and just make it work. You can always add an SLOG later if you find you may need it, but spend the money for the power loss prevention or forget about using an SLOG.

Makes sense.

A quick question though. Will there be any benefit if i use SATA SSDs as a data vdev and then use Optane 900P as a metadata special vdev?

 

[NugentS]

A metadata special is pool critical so will require the same level of resiliancy as the pool. Lose the vdev, lose the pool
L2ARC is not pool critical - failure does not fail the pool
SLOG is not pool critical - failure does not fail the pool

I use a metadata vdev on an HDD pool. Much less point on an SSD Pool

 

[Fastline]

A metadata special is pool critical so will require the same level of resiliancy as the pool. Lose the vdev, lose the pool
L2ARC is not pool critical - failure does not fail the pool
SLOG is not pool critical - failure does not fail the pool

I use a metadata vdev on an HDD pool. Much less point on an SSD Pool

Got it.

A few more questions:

1. What should be the size of the Snapshot vdev?
2. The Snapshot vdev should be an SSD or can I use HDD?
3. Have never used the Snapshot feature on the TrueNAS side. Is it similar like Time Machine on Mac?

 

[NugentS]

Whats a snapshot vdev - no such thing. Snapshots use space wherever the data is

 

[Fastline]

Whats a snapshot vdev - no such thing

My bad. I meant the Snapshot drive. xD

 

[NugentS]

Again - no such thing

 

[Fastline]

Again - no such thing

Damn. Location where you store the Snapshots. Is that clear now or i still need to learn more before asking the questions here?

 

[NugentS]

Again - no such thing. A snapshot is a point in time. when you create a snapshot you freeze all existing data on the pool. If you then add or change data this is written to a new area on the disk, but the old data is not deleted, Thus you can return to the point in time represented by the snapshot. A snapshot in itself uses no (well a tiny bit) of disk space. But changes after add more disk space as previous data never gets deleted.

The never gets deleted means that you have to prune snapshots from time to time - otherwise you WILL run out of disk space

 

[Fastline]

Again - no such thing. A snapshot is a point in time. when you create a snapshot you freeze all existing data on the pool. If you then add or change data this is written to a new area on the disk, but the old data is not deleted, Thus you can return to the point in time represented by the snapshot. A snapshot in itself uses no (well a tiny bit) of disk space. But changes after add more disk space as previous data never gets deleted.

The never gets deleted means that you have to prune snapshots from time to time - otherwise you WILL run out of disk space

Sounds interesting. But is there any option where i can store the Snapshots in a specific Drive, other than the metadata, SLOG, L2ARC or the data vdev?

 

[NugentS]

So If I have a 100MB Pool with 50MB of data on it.
I delete 10MB - pool used is now 40MB
I add 5MB - pool used is now 45MB

I now snapshot - Pool used is 45MB
I remove 10MB - Pool used is 45MB - the data is still there
I add 10MB - Pool used is 55MB
I delete all the data - Pool used is 55MB
I add 50MB of new data - just ran out of space as total pool used is 105MB

 

[NugentS]

Sounds interesting. But is there any option where i can store the Snapshots in a specific Drive, other than the metadata, SLOG, L2ARC or the data vdev?

Yes. You can replicate a snapshot to another pool (as long as it has enough disk space). That is the whole basis behind snapshot replication

 

[Fastline]

Yes. You can replicate a snapshot to another pool (as long as it has enough disk space). That is the whole basis behind snapshot replication

That's cool. Seems like i will have to test in on a local test machine to understand it more. Thanks.

 

[Davvo]

Snapshots reside on the pool. This can be manually changed iirc.

 

[NugentS]

Snapshots reside on the pool. This can be manually changed iirc.

Snapshots are an integral part of the pool - they can be replicated off the pool, even deleted from the original pool - but the snapshot, in its first instance, is always part of the original pool. A snapshot is just a set of pointers to data on the pool. When you copy a snapshot to another pool, you aren't copying the pointers, you are copying the data & the pointers to another pool.

Is pointers the right term here?

 

[joeschmuck]

This started to make me laugh, I find comical relief in strange places. Sorry, just been one of those Really Long days at work.

 

[NugentS]

Glad to be of assistance

 

[Patrick M. Hausen]

Is pointers the right term here?

I think so. So when you clone a snapshot are you creating pointer sisters?

 

[joeschmuck]

pointer sisters?

LOL, another good one!