RAM requirement still 1GB per 1TB even if mirroring?

[Rathseg]

Seeing the part in the guide about wanting roughly 1 GB RAM per 1 TB disk space, does that still hold true if you are planning to mirror drives instead of doing a RAIDZ setup? Not being a filesystem expert, I don't know if simplifications can be made that would reduce RAM usage due to the fact that the drives are duplicates of each other and there is no parity to calculate since I am not doing RAIDZ.

For an example:

2x - 6 TB drives mirrored with ZFS => 6 TB usuable space, 12 TB actual disk space.

Would 8 GB RAM be sufficient or would I want to bump up to 16 GB? Or put slightly differently, is it the 6 TB of usable space that is important for this guideline or the 12 TB of actual space? I understand that 8 GB could still be cutting it close if 8 GB does work when accounting for the RAM needs of the OS, write-caching, compression, and other services. I'm mainly trying to get a better feel for the effects of mirroring and usuable space vs actual space, with a bit of consideration to future RAM needs if I later upgrade to 4 drives instead of two.

Thanks.

 

[cyberjock]

The "savings" due to not calculating parity is trivial. So no, you can't really argue that you can go with less RAM because you did mirrors over RAIDZ. :P

Some argue you exclude redundancy, others say you include it. The bottom line is as long as you have 8GB of RAM and performance is satisfactory then you have enough RAM. But it's better to not skimp on RAM. If you upgrade to 4 disks you will almost certainly need 16GB of RAM.

 

[jgreco]

Put simply, the 1GB/1TB thing is merely a rule of thumb to get you in the right neighborhood. There is no mathematical formula to provide a correct amount of RAM. Some users could get by on a little less, others might need significantly more to get good performance. The guide doesn't specify whether it is 1GB per 1TB of: a) raw disk space, b) available space, or c) used space. When I rewrote all that for the 8GB minimum bump, I deliberately avoided specifying because there isn't a magic formula, but people who might benefit from more memory are typically expecting to have to "go big" and having 1GB/1TB of raw space usually doesn't seem onerous to them, while home users will usually take a more conservative guess.

The best thing to do is to get high density sticks (i.e. for Xeon E3 do NOT get 4GB sticks) paying attention to the 1GB/1TB rule, and then if you think the performance isn't up to expectation, increase. If you're running a busy departmental fileserver, go bigger to begin with.