Not sure if my ARC Hit Ratio is good or bad

[file_haver]

Hi everyone,

This server has the specs in my signature and 90% of the time is only running Plex server and nothing else so for all intents and purposes it is a media server.

This is what it looks like pretty much any time i check it while it's doing Plex, I am a little surprised it's not more consistent and i'm not sure if this is an indication i need more ram or a l2arc ssd. I do have a non used 64gb ssd right now.

I basically understand that the arc hit ratio tells you how frequently the arc has the data the system wants versus doesn't but i would have thought it would be a little more consistent. During the screenshot the spikes are when a new episode starts from one of the Plex clients. At the time of taking this screenshot it only has one client connected.

 

[HoneyBadger]

Putting aside the hit rate, 8GB is considered the "bare minimum" for FreeNAS alone without plugins, and you've added Plex on top of that. So I would say you need more memory just for stability/safety reasons. Are you using your swap space at all?

With regards to your hit rate; your use case is as a Plex server, so unless you plan to rewatch the same episodes over and over again, your hit rate is always likely to be rather poor, barring any gains seen from prefetching; I wouldn't sweat that number much at all.

 

[file_haver]

Thanks so much. My swap space is actually never getting used ever and has not been once since i set up this server.

To be honest, for the first few months I ran this FreeNAS server as a combo Plex and Samba server (it's been upgraded a few times) I ran it on 4GB and never hit swap space then either.

 

[file_haver]

Hi everyone, back to bump this thread again.

I've upgraded the machine from 8gb to 16gb RAM and done some trials with and without a 64gb SSD as L2ARC but wanted a little help making sense of the results i have. First of all even though performance was fine with 8gb RAM, the system seems much happier with 16GB RAM as the ARC seems to be more stable.

At the end of the day it seems like i could live without a l2arc and be just fine. It seems to improve my ARC hit ratio on average but it doesn't seem that different with the L2ARC SSD enabled or disabled. However i don't want to blindly accept that L2ARC is good for a Plex server without understanding why i might be seeing this sort of improvement, even if it doesn't seem too drastic.

This test was done by playing 2 random shows off Plex to 2 different LAN devices. Between each colored section I closed all the Plex clients and started completely different shows. The light blue tinted part at the beginning is no L2ARC. The purple tinted part in the middle is with L2ARC activated. The red tinted part at the end is with L2ARC activated and a 3rd client connecting to play a different video.

Unfortunately I don't have a screenshot of how the graphs show 3 streaming devices with no L2ARC, but since that's how my setup has been for several months, I'm used to seeing between 0 and 30ish% hit rate with a few outlier spikes here and there and a ARC size of nearing 5GB.

Interesting to note that by the time i add a third client, the L2ARC really starts picking up in terms of size, it goes from around 400-500mb to just over 1GB.

Maybe L2ARC isn't a bad idea for a plex server after all? Seems like if one lets the episodes play one after another like usual, the server does a pretty good job of expecting what to serve next, but like i said, i'd rather really understand what's going on as opposed to looking for what i want to see.

One last question.... Why does my arc hit ratio look like this when the plex server isn't being used? Seems weird that it is so blocky, when before I had a l2arc it would spike between 0 and 100 a lot. Dont really understand why it's different with the l2arc active.

 

[sretalla]

L2ARC isn't recommended on systems with less than 32GB of RAM (64 and up preferred).

The allocation tables for L2ARC are stored in ARC, so will actually make your performance worse since you have less ARC available.

 

[toadman]

I would start with "what problem are you trying to solve?" For example, if Plex is working, does it really make a difference what the ARC hit rate is? (I think the answer is "no". Don't worry about it. :))

If there are performance issues or stability you are trying to solve, looking at the architecture and resources are natural places to start.

The ARC is basically a read cache that is balancing "frequently used data" (frequently used data is likely to be used again) and "recently used data" (recently used data is likely to be used again). A media file is neither of those things - i.e. you are not going to re-read blocks of the file. Unless perhaps you are doing some close proximity "DVR" rewind or something.

 

[Constantin]

Preliminary testing suggests adding 500GB L2ARC to a 64GB RAM FreeNAS Mini XL, cut my rsync times in half once the L2ARC was populated. I haven't tuned it yet, so I will run a few more trials to see if the improvement is consistent or not. The experience also underscored for me how organizing the content can make a big difference.

 

[file_haver]

I would start with "what problem are you trying to solve?" For example, if Plex is working, does it really make a difference what the ARC hit rate is? (I think the answer is "no". Don't worry about it. :))

If there are performance issues or stability you are trying to solve, looking at the architecture and resources are natural places to start.

The ARC is basically a read cache that is balancing "frequently used data" (frequently used data is likely to be used again) and "recently used data" (recently used data is likely to be used again). A media file is neither of those things - i.e. you are not going to re-read blocks of the file. Unless perhaps you are doing some close proximity "DVR" rewind or something.

That is a great point so thanks. Point taken. I was trying to save my drives from constant usage but now i understand better what the l2arc is there for.
I also see that if i want to use an l2arc i should focus more on having more RAM first and probably have a drive bigger than 64gb for l2arc purpose.