Adding SSD L2 ARC after creating a RAIDZ2 pool

[doublehelix]

I have FreeNAS-8.3.0-RELEASE-p1-x64 (r12825) installed in NAS with RAIDZ2 array of 6 disks. I wanted to add an L2ARC after I pulled an SSD out of my desktop. But when I select the SSD using volume manager in the GUI, there is no "ZFS extra option" to add the SSD as an L2ARC. Here is the screenshot.

I saw a thread which was started a while back on this issue... http://forums.freenas.org/showthread.php?5717-How-I-added-a-cache-drive-to-an-existing-pool
Is this still the solution for this problem or am I doing something wrong?

Appreciate any help!!

 

[William Grzybowski]

You're doing something wrong.

_DO NOT_ select the disk in the "member disks" field, it will show up and the bottom to chose as a cache, additionally select the "volume to extend" field.

 

[cyberjock]

I can't check this myself right now but you have to choose the volume to extend. Then it appears.

 

[jgreco]

I can confirm that it can be done through the GUI, because that's how I did it. William's directions sound approximately correct. It'll be obvious when you're doing the right thing. Select the pool under "volume to extend", leave "volume name" blank.

 

[doublehelix]

William, noobsauce and jgreco - thank you so much!

I left the volume name blank, selected the volume to extend and did not select the disk in the "member disk" field. It worked!
Thanks again!

 

[nvader]

is it correct that adding an L2ARC to your zpool will cause a write performance hit?

 

[cyberjock]

is it correct that adding an L2ARC to your zpool will cause a write performance hit?

WOW. Talk about a question with 1000 yes and nos. Yes and no. The short and sweet answer for new people is that if you don't know if a ZIL or L2ARC will help you then you most likely have zero use for it. Having an L2ARC consumes system RAM that could be served for the ARC or the RAM write cache. So yes, adding an L2ARC can hurt system performance in some situations.

My opinion, since you are asking this question you probably have zero use for an L2ARC and ZIL. I don't use it in any of my servers and I get great performance. :)

 

[nvader]

WOW. Talk about a question with 1000 yes and nos. Yes and no. The short and sweet answer for new people is that if you don't know if a ZIL or L2ARC will help you then you most likely have zero use for it. Having an L2ARC consumes system RAM that could be served for the ARC or the RAM write cache. So yes, adding an L2ARC can hurt system performance in some situations.

My opinion, since you are asking this question you probably have zero use for an L2ARC and ZIL. I don't use it in any of my servers and I get great performance. :)

I appreciate the response to my question, I guess what i meant was is it "still" correct. since i'm pretty sure i read about the possible performance hit in your guide. but things change. your guide/info could be a little dated. so i was just looking for a confirmation, with the price of SSD's today, it certainly couldn't hurt to check it out..

 

[cyberjock]

It could be a little dated? Did you not see the part that says updated "This presentation is accurate oas of FreeNAS 8.3.0-RELEASE (October 26, 2012)? I'll keep it up to date as long as I continue to use FreeNAS.

 

[nvader]

It could be a little dated? Did you not see the part that says updated "This presentation is accurate oas of FreeNAS 8.3.0-RELEASE (October 26, 2012)? I'll keep it up to date as long as I continue to use FreeNAS.

clearly I didn't. Sorry if I offended you.

 

[doublehelix]

FWIW...I have not noticed any performance hit so far just looking at the write speeds using windows file transfer. I have not done any extensive benchmarking looking for before and after performance differences. Specs - Processor i3 3225, 16GB RAM, 6*3TB drive in RAIDZ2, 64GB SSD LR2ARC, Intel NIC.

 

[cyberjock]

That's why I said the answer has 1000 yes and nos.

You probably aren't seeing much of a performance boost using an L2ARC either. :)

The L2ARC is something that can be a complete waste of resources. For home setups(and especially streaming movies) it is a complete waste of money.

 

[doublehelix]

I have not checked the read speeds. For media serving, I do not really care about the possible extra speed advantage. If a file bigger than system ram and smaller than the L2ARC is accessed multiple times over 2 hours, is it not better that the file is served from cache than from the hard drive platter, thereby reducing unnecessary wear and tear?

 

[cyberjock]

I have not checked the read speeds. For media serving, I do not really care about the possible extra speed advantage. If a file bigger than system ram and smaller than the L2ARC is accessed multiple times over 2 hours, is it not better that the file is served from cache than from the hard drive platter, thereby reducing unnecessary wear and tear?

While your thought process is sound I just don't think that resembles reality. Not to mention that if you really think that those 2 hours are going to make a difference on a drive that ideally will have a 20000 lifespan you're trying to make something better that will have just about 0% chance of making the drives last longer. Plus, whats to say that having an L2ARC doesn't increase wear and tear by trying to cache huge chunks of files you will never access?

I know from personal experience that running a drive 24x7 is far better for hard disks than shutting them down at night. I can't explain why this is so, only that it is. Other senior forum members have noticed the same result with leaving drives on all of the time. I think it has to do with the temperature changes as the drive heats up when powered on and cools down when its off but thats just my best guess.

For example, pre-SSD I used to have 3 desktops in my house. I'd shut them down when not using them to save electricity. I could expect at least 1 drive to fail within 12-18 months like clockwork. RMAs sucked and I enjoyed the 5 year warranty. The server I have today has 24 drives in it. By the same rules I should expect a drive to fail very frequently. Instead I've had 1 failure in over 3 years of continuous use. My previous 3 generations of 24x7 always on servers also had amazing lifespans despite being on 24x7.

So honestly, if you really want to tell yourself that you are saving wear and tear go ahead. I'd never ever buy an L2ARC or ZIL hoping to save on wear and tear. I WOULD and DO tell people that if you want a long lifespan on your computer you should just leave it on all the time and never let it sleep. What I definitely do however is buy at least 16GB of RAM for every server. I haven't built a server with less than that yet. ZFS seems to only read 32MB into the "future" on any file access(not sure how it works for L2ARC) and the write cache in RAM will help turn a bunch of small writes into sequential writes. I also like to keep things simple as it makes troubleshooting a problem far easier as well as having less things that can go wrong.