Design consideration for SLOG

[curtii]

Hello everyone,

Planning out a new FreeNAS build that I'm quite excited about. Here's the lineup I've got planned so far:

Chassis: Supermicro SYS-5019S-WR (Includes the Motherboard and PSUs)
Motherboard: Supermicro X11SSW-F
CPU: Xeon E3-1240V6
RAM: 16gb Micron MTA18ASF2G72AZ-2G3A1
Boot Disk: 2x 64gb SuperMicro SATA DOMs
Storage Drives: 4x HGST Ultrastar 7K4000 (These are being repurposed from a different system. Otherwise, I'd be going with Seagate Iron Wolfs)

What I'm considering now is adding a SLOG device to improve performance. My connection options are 4x standard SATA connectors, and 1x M.2 slot. The drawback to the SATA connectors is that there's no actual spots in the case to mount additional drives. I'd have to rig something up for them to be fastened into the space that would normally be taken up by PCIe cards.

I haven't yet confirmed what the best candidate for an M.2 SSD would be, but I'm thinking something like a 250gb Samsung 970 EVO NVMe

The first consideration I'm struggling with is the importance doing a mirrored VDEV for the SLOG. Given that this will be enduring quite a bit of writes, I'm a bit concerned about the durability of SSDs in the long-run. If that concern is legitimate, what I could do is get 2x Micron 5200's (Which have pretty insane durability for SSDs)

After thinking on this, I realized another option which would be to use the two SATA DOMs as the (mirrored) SLOG, and the M.2 as the boot disk. I'm thinking that's probably less appealing overall, but worth considering anyways.

I'm going to keep researching this, but wanted to put this one to the forum and see if anyone had strong advice one way or another on this. Thanks in advance for any input!

 

[Chris Moore]

What is your system for? Most people don't need to use a SLOG device at all. Why do you think you need it?

Sent from my SAMSUNG-SGH-I537 using Tapatalk

 

[curtii]

The NAS will serve quite a few functions. I run a small cluster of Proxmox VM hosts, and this will provide shared storage for the virtual machines to use. I'll be using it as a cloud backup destination, and I also host a Nextcloud file server. There are some databases involved with some of the applications, such as Nextcloud and the Icinga monitoring server I run. I'm also trying to build this to scale nicely as I add more applications to my environment.

It'll be pretty varied in it's use, but my thought to put a SLOG in there wasn't out of necessity. I just thought it may be a relatively inexpensive way to improve performance.

 

[Chris Moore]

It'll be pretty varied in it's use, but my thought to put a SLOG in there wasn't out of necessity. I just thought it may be a relatively inexpensive way to improve performance.

Some of those tasks may benefit from SLOG. The thing that it is most applicable to is accelerating sync write. Anything that is not a sync write will not utilize the SLOG at all.

There has been a lot of discussion around the use and what is best to use. You should look into these discussions:

Testing the benefits of SLOG using a RAM disk!
https://forums.freenas.org/index.ph...s-of-slog-using-a-ram-disk.56561/#post-396630

Testing the benefits of SLOG
https://forums.freenas.org/index.php?threads/testing-the-benefits-of-slog-using-a-ram-disk.56561

SLOG benchmarking and finding the best SLOG
https://forums.freenas.org/index.ph...-and-finding-the-best-slog.63521/#post-454773

 

[Ericloewe]

I haven't yet confirmed what the best candidate for an M.2 SSD would be, but I'm thinking something like a 250gb Samsung 970 EVO NVMe

If you do find that an SLOG would be beneficial, that's a poor choice. It has no power loss protection, making it semi-useless as an SLOG, and it's tuned for consumer workloads unlike those it's likely to encounter as an SLOG.
As for the cheaply part, that's rarely the case with SLOG devices.

 

[curtii]

Thank you all for your input, this has definitely helped guide me to understanding all of this better.

One question I haven't been able to find an answer to, is it important that SLOG be a dedicated device? I'm wondering if I could make a partition on my boot device to use for SLOG. (of course, the boot device would then need to fit the SLOG hardware requirements, such as what Ericloewe said).

I also haven't determined the importance of mirroring the SLOG. Right now, that's a big factor for if I even continue considering using SLOG. One high-end NVMe drive may be in the budget, but I really don't think two is likely to happen.

 

[Ericloewe]

If you have something really good, you can probably get away with sharing the boot device with it - but you'll need to manually hack that together.

As for mirroring, it's as important as those few seconds of data that may be in the ZIL when the server crashes. Not important at all? Sync=disable. Important, but not "two high-end NVMe drives" important? Don't mirror. Your business model hinges on that data absolutely never being lost? Mirror it.

 

[HoneyBadger]

The question to answer first is "do I really need an SLOG device?" Most home users don't - even those running VMs will often opt for "I'll take regular backups, and I'm willing to roll back to a previous snapshot or external backup from yesterday." It's an acceptable risk to them; but if it's not to you, then read on.

is it important that SLOG be a dedicated device?

Yes. An ideal SLOG device has characteristics that are vastly different to the rest of the pool - it needs to be able to service writes incredibly quickly with consistent low latency, but really doesn't need to be read from ever unless there's an unexpected failure.

Pre-partitioning the boot device is a bit of a mixed bag as you could wind up with some strange issues during an upgrade or migration. That's about the only workload I'd consider sharing with SLOG.

I also haven't determined the importance of mirroring the SLOG. Right now, that's a big factor for if I even continue considering using SLOG. One high-end NVMe drive may be in the budget, but I really don't think two is likely to happen.

Mirrored SLOG is about two things - risk reduction and performance guarantees. If you absolutely can't afford to lose the data in flight, even in case of a simultaneous power and SSD failure - then mirror it. If your workload is sustained, and the loss in performance if your SLOG blows up would be crippling - then you probably want not only a mirrored but also a hot-swappable SLOG, so that would be U.2 bays rather than M.2 or PCIe slot cards.

In your case, since I'm assuming the SLOG is a "nice to have" - I'd suggest using an Optane Memory 32GB - benchmark results here: https://forums.freenas.org/index.ph...inding-the-best-slog.63521/page-4#post-484151