Feature Request: SLOG sharing boot pool disks

I missed that you have two boots that you want to partition and then have a mirror.

I thought you wanted to partition another SSD and then add both partitions as a mirrored slog.

You can do what you want. It’s unsupported. Requires command line hacking (and in fact installer hacking), and I don’t think doing this will become supported anytime soon.

AND it’s unlikely that the boot disk is a suitable device for use as a slog.

You do realize you need power loss protection on the SLOG, if you actually want the benefits of sync writes, right? “Free space” is not even on the list of relevant criteria when choosing a suitable SLOG device, given the relatively tiny needs in terms of space.

If you don’t care, then just disable sync writes and be done with it. No performance penalty, and no illusions.

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I specifically indicated that I do not want to follow an unsupported path and that I was asking about why this can or cannot be a feature in TrueNAS (for a small edge server with one data pool).

The boot devices are not very busy and have lots (100s of GBs) of available space. They are SSDs while the data disks are HDDs. The documentation says that

You can further improve ZIL performance by using a dedicated vdev called a separate intent log (SLOG). A SLOG moves the ZIL to a dedicated SSD(s) instead of a section of the data disks to function. This can provide a large benefit due to lower latency of a SLOG on SSD(s) vs data disks.

Why do you think “it’s unlikely that the boot disk is a suitable device as a slog”?


I do but I am missing your point about power loss protection. The SSDs are non-volatile storage. And, the server as a whole is protected by a UPS giving it time to shutdown in the event of a prolonged outage.

And, yes, I do want to get the benefit for synchronous writes. The documentation says that

Disk latency and write endurance capability are the primary concern for slog devices. You might need multiple striped slog devices to reach write endurance and the synchronous write throughput needs of each slog device. Combined SLOG write throughput should be higher than the planned synchronous write throughput of the pool.

So, using a CMR SSD would satisfy my requirement.

Because you said they were “cheap” 2TB devices.

A suitable SLOG needs reliable high speed small block sequential sync write performance and very high endurance. Every sync byte received by the server is written to the SLOG using a sync write before being acknowledged to the client.

That means either enterprise style power loss prevention support (flash, power backed ram) or something else magic (Optane)

Do your boot disks match that?

That’s an abstraction. SSDs have caches and they’re unpredictable (to us they are, anyway).

Power outages are not the main concern, because the other end of the connection is probably also going down in the middle of doing something - literally anything else that kills your server is: panics, hardware failures, etc.

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Conventional Magnetic Recording Solid State Disk.

Well. Yes. HDs do work. As long as they implement sync write support correctly. But they are slow.

This is why SLOG was invented. To make sync writes faster than HDs.

The original Sun implementation used battery backed RAM disks iirc

By “cheap”, I meant relative to retail price - i.e., I got a good deal. Perhaps, “inexpensive” would have been a better word.

I am using Samsung Enterprise SSDs. If these are not sufficient, in your opinion, I am open to recommendations for better quality SSDs.

Do all enterprise SSDs have this caching issue? If true, why does the documentation suggest that they would be suitable when other failure concerns are strongly called out (e.g., be sure to use ECC memory)?

How does one check that sync write support is implemented correctly on the SSD specification (i.e., what does one look for)?

Enterprise SSDs usually have the PLP support required to have good sync performance (it’s one of the differentiators between enterprise and consumer)

You can benchmark one of the drives to see.

(Or you might even find a benchmark)

Thank you for the reference. But, back to my original question.

Assuming that I have a pair of enterprise SSDs with the PLP support required to have good sync performance and given that enterprise SSDs are always significantly larger than are needed for a boot pool

In the same way that TrueNAS will configure a 16GB swap partition on the mirrored disks that are serving the boot pool, is there a downside to have TrueNAS also create a similar, additional 16GB partition as an option to serve as a mirrored VDEV to host a SLOG for the main pool?

Relative to the PLP support, I found this thread on the old forum.

And, reviewing this whitepaper from Samsung, I should have considered the SM863 for write-intensive applications. The 870 Evo does not have (anywhere documented) PLP support.

Looking at the latest in the Samsung Enterprise SSDs, the PM1733 NVMe SSD is the one to consider.

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LOL, in what crazy world is the EVO series considered enterprise? Not even Samsung themselves market them as enterprise. I cant even find one mention of the word enterprise on that page. Unlike this page here where they clearly mention it right in the heading.

EVO is their budget line. I thought you’d at least choose the PRO line (which is also not enterprise), but at least a step up. You need to go to at lesst the (S/P)M-863 series to get entry level enterprise.

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Please no.

  • SLOG duty is a write-intensive application that gnawls at a drive’s endurance;
  • You require said drive to perform to its best in order to not further slow down your pool.

i get the point though. I have also have been wondering if there was anything i could to make use of the free space on my mirrored boot 240Gb SSD’s but so far, i haven’t really found anything worth the hassle. I have considered moving my jails the SSD’s but that would get me into trouble if my boot pool crashes and i would need to reinstall.

In the end, i do not want to get creative and endanger the system as it is running very well and the potential performance gain would not justify putting the system at risk.

Yes. Big error on my part.

I thought CMR was all that mattered. I understand now that PLP support is also required. The drives have already been replaced.

Now that the type of drives for my boot pool have been addressed, are you able to answer my original question?

According to the documentation, a SLOG should be on faster drives than the drives serving the pool. If not enterprise SSDs (as the docs suggest), then what?

Maybe wrong ping? I replied to the first post of this topic.

Going back to one of the original questions:

Is it possible in a supported way, (or will likely be added), to share a boot device with SLOG?

The answer is no.

The reason is simple. iXsystems developed / is developing TrueNAS, (Core & SCALE), for the Enterprise Data Center market. That market requires reliability more that cost saving features of shared function storage devices.

This idea comes up several times a year, (more so since TrueNAS SCALE was released). And the answer has not changed.



@ddaenen1 actually gave the answer just above: Partitioned drives give rise to multiple failures when they fail as a whole, and then replacement is a complicated issue. TrueNAS is designed for data safety and reliability above all other considerations.
Using a full drive for boot allows the boot drive to a be a replaceable commodity while keeping data safe.