L2ARC Consideration

[DanPrs]

I'd like to know how to calculate SSD size, for the following configuration. Mind you, I'm rebuilding the entire system so I can make any necessary changes. Currently offloading all data to spare drives to allow me to increase the number of drives in the pool.

5x 3T HDD
16GB ECC RAM (ITX Motherboard does not allow me to add any more)

Storage looks something like:

3T of Media available to Plex.
1.5T for VMs (VMWare and Hyper-V hosted on separate hosts, some iSCSI, some shares)
The rest is ~1T PC backups, jails and some other random data.

 

[BigDave]

The forum search feature is your friend ;)

This may be of use and there were many others that popped up on the search.
The linked thread is old but the reasoning still applies...

https://forums.freenas.org/index.php?threads/formula-for-size-of-l2arc-needed.17947/

 

[tvsjr]

With that limited RAM, you shouldn't even be considering L2ARC. It'll make things worse, not better.

For your VM storage, you DO need an SLOG device. Plenty of threads on this topic. You want a high-endurance enterprise-class drive, underprovisioned... 16GB is more than enough, but you'll want a larger drive for better wear leveling/etc.

 

[DrKK]

Yeah, let me second that, with paltry RAM (16GB), you have a 30% chance of doing nothing, and a 69.9% chance of harming your performance, with an L2ARC.

 

[tvsjr]

Yeah, let me second that, with paltry RAM (16GB), you have a 30% chance of doing nothing, and a 69.9% chance of harming your performance, with an L2ARC.

0.01% of improving performance? That's more optimistic than I've come to expect from you :D

 

[Ericloewe]

0.01% of improving performance? That's more optimistic than I've come to expect from you :D

Well, compressed ARC changes things and may make this a somewhat less silly idea than it used to be.

 

[tvsjr]

Well, compressed ARC changes things and may make this a somewhat less silly idea than it used to be.

Well, I fulfilled my learning-something-new quota for the day. Compressed ARC? One wonders how adding compression in front of something that is supposed to be as close to instantaneous is a good idea...

 

[Ericloewe]

Well, I fulfilled my learning-something-new quota for the day. Compressed ARC? One wonders how adding compression in front of something that is supposed to be as close to instantaneous is a good idea...

It's the age-old latency vs. capacity tradeoff. The extra latency of LZ4 decompression is small and much easier to bear than having to grab more stuff from disk. There's also a small (~100MB) section that is not compressed.

 

[Ericloewe]

Note: Don't try GZIP and compressed ARC on a server you care about. I'm not sure anybody's tried, but it's bound to be a nasty experience.

 

[Stux]

It's the age-old latency vs. capacity tradeoff. The extra latency of LZ4 decompression is small and much easier to bear than having to grab more stuff from disk. There's also a small (~100MB) section that is not compressed.

Trading off CPU usage to get more data out of the same amount ARC/L2ARC and thus prevent reading data from disk.

Same reason ZFS+compression is the default.