simple RAM question

[MrHands]

hi,

i know from reading the guides on here that 1GB of RAM is generally recommended per 1TB of storage, quick question: does that 1TB include the redundancy drives? so if i have a RAIDZ2 setup with six 4TB drives which gives me 16TB of storage space, does that mean i should have 32GB of RAM? or does it include all 6 drives which would mean 40GB of RAM (i hope not as my my board only has 4 DIMM slots and i already have 2x8GB and the 16gb sticks are insanely priced).

thanks for any advice!

 

[Ericloewe]

It's a rule of thumb. It's meant to be flexible.

That said, your math is off - 6 * 4 = 24, not 32GB (or 32GB and not 40GB, if you're adding the 8GB minimum).

In any case, 32GB of RAM should not be a problem and 16GB will probably be fine.

 

[Stux]

It's intentionally vague.

16-24TB so 16-32GB.

Depending on you.

 

[MrHands]

It's a rule of thumb. It's meant to be flexible.

That said, your math is off - 6 * 4 = 24, not 32GB (or 32GB and not 40GB, if you're adding the 8GB minimum).

In any case, 32GB of RAM should not be a problem and 16GB will probably be fine.

Thanks.. Its been a long day, for some reason I was thinking of 2gb per 1tb.

If I try and get by using 16gb for the 20tb of data, what issues should I be on the look out for if it turns out to be insufficient? Just slow performance?

 

[Ericloewe]

Just slow performance?

Yeah, but you probably won't see it.

 

[Dice]

Just slow performance?

If you have mostly squadzillion amounts of small files, RAM plays quite the role during large transfers.
I wouldn't say that more RAM makes the system <faster> but, it will prevent the system stalling out during transfers. Which is a user experience that sux enough to immediately buy more RAM regardless of budget. I've been there. Started at 16GB. A couple of weeks later I was at 48GB wishing I had gone for 16GB memory sticks.

 

[scwst]

Just slow performance?

Note there is a difference between async and sync writes. The first writes stuff to RAM, then the server says "got it" and does the actually writing to the drive later, so it feels super fast, but does eat RAM like popcorn. The second (slightly simplified) waits until the data is actually on the drive before sending back the "all okay". This is to keep your data safe but can be slower.

So you'll need to check which one you are going to have. In my case, I'm using NFS (FreeNAS serves to Xubuntu), which does sync writes. In theory, this would probably benefit from a SLOG, but honestly I can't be bothered for my little home server (I might get a 32 GB SSD at some point just to test it). AFAIK SMB uses async writes. Also, remember that your network is probably going to be the bottleneck anyway.

 

[Ericloewe]

Note there is a difference between async and sync writes. The first writes stuff to RAM, then the server says "got it" and does the actually writing to the drive later, so it feels super fast, but does eat RAM like popcorn. The second (slightly simplified) waits until the data is actually on the drive before sending back the "all okay". This is to keep your data safe but can be slower.

Neither uses more RAM. The ZIL is never read from unless something goes wrong.

 

[scwst]

The ZIL is never read from unless something goes wrong.

No, but it is written to, which still takes time and can produce interesting effects if it is on a SLOG. For instance, small SSDs (say, 32 GByte) have quick read speeds, but some have horrible, horrible write speeds.

 

[Ericloewe]

No, but it is written to, which still takes time and can produce interesting effects if it is on a SLOG. For instance, small SSDs (say, 32 GByte) have quick read speeds, but some have horrible, horrible write speeds.

Sure, but it's got nothing to do with RAM.

As for reasonable SSDs, client SSDs aren't (well, almost never) tuned for the constant writes scenario that SLOG presents, so performance is crap. More importantly, without power loss protection, those SSDs are just paperweights that slow everything down, because they won't be able to commit the writes they were given, making their usage as SLOG devices rather silly.