FreeNAS: SLOG and L2ARC for all SAS SSD pool?

Computer Hardware & Server Infrastructure Builds
#1

I am upgrading our production FreeNAS server with a new SAS SSD pool to run all our IO intensive VMs. Would it be worth it to use a SLOG device at this point?

I plan on using 6 ssds in a 3 mirror vdev. 3 proxmox hosts will have 1 (maybe 2) 10gbps links to our switch, while freenas will be connected to the switch with a 40gbps link. NFSv3 will be the protocol.

I have also been contemplating on using 4 NVME drives in a 2 mirror vdev but from what I’ve researched NVME and zfs are not really able to hit the insane IOPS they’re usually capable of. SAS 12gbps seems like it’s rock solid. I’m using an LSI 9300-8e so that should have the throughput I need. Plus, we really like the idea of being able to hotswap our SAS SSDs which can’t happen with nvme drives on riser cards.

Any thoughts? I’d appreciate any advice as this is my first adventure into tiered data storage rather that just firing up a big fat NAS to dump backups to.

#2

You should bench the build and find out. If the throughput and latency are acceptable for your use case then no, if it is not then yes.

#3

Not using NVME so you can hot plug sounds like a good idea although as you know you will be sacrificing some speed for that benefit.

My logic would be that if the budget allows you to add the SLOG / ZIL then put it in as it isn’t going to hurt.

#4

If you are going to put in the caching, you will probably want that to be nvme drives or Optane drives, something fast.

#5

I wasn’t 100% awake the first time I replied so I thought I would add some clarification.

If you are going all SSD one may start to question the use of the SLOG and depending on the work load and the drive they are using they might be correct.

Most folks go, “I am using 6G SSDs which are good for about 500MBs on average at that speed do I really need a SLOG” but they are forgetting about PLP(Power Loss Prevention). There is a reason on the IX Systems forums you keep seeing the same recommended SLOG drives over and over again and it because they have performance and PLP. It is not just about the throughput but also about protecting your data. How important is PLP depends of your workload if your data is relatively static you could say I got good backups I don’t care. If you are running volumes of transactional data you may very well care about the potential loss of data and between the time of corruption and the last backup was taken.

If all your SSDs have PLP then the use of a SLOG really comes down to performance, but most shops won’t do that because of the cost difference. A Samsung 883 DCT 3.84TB with PLP is about $700 each and a Samsung 860 DCT 3.84TB without PLP is about $560. In a 24 Bay chassis could be as much as a $3,360 difference in price for otherwise equal performance.

You also can’t just put a 883 DCT as a SLOG because the 860 DCT pool will out perform the single 883 DCT creating a bottle neck at the SLOG. With NVME being the current widely available king of storage it is the usual pick for SLOG drives. There is however another option is often overlooked which is the 12g SAS SSD which about 4x faster than a 6g SSD.

The question

To SLOG or not to SLOG

It is a simple yes or no question, that one cannot answer without all the details.

***Note I am not suggesting the use of any hardware listed in this post for use they have not been researched for production reliability and were only used to illustrate a point.