Want to buy a SSD for performance but use it for ZIL or Cache ?

[djlax152]

Hi guys, I am trying to boost the performance of my Freenas box I am a novice so I tried to read about how the ZIL and Cache work with ZFS but i am still a little puzzled as to which would probably give me the most performance gain. I use my freenas box to host "my documents", Stream my movies and some FTP use. Currently i have a RAIDZ with 4x 1TB drives, 8GB DDR3 RAM. I have 1 CFIS share that I use to access all of my files.

So my questions are based on what i am doing with my Freenas if i buy an SSD I am going to benefit more by using this drive to host the ZIL or use it as a cache drive? Also how big of an SSD do you think i should get? anyone have any thoughts or input??

I read here and it sounds like Putting the ZIL on a separate drive could really help but it seems like you'll only notice a big difference if you have a lot of synchronous writes. They have a Utility to see how your ZIL looks but i don't know how to use it with Freenas.
(Article)
http://constantin.glez.de/blog/2010/07/solaris-zfs-synchronous-writes-and-zil-explained
(Utility)
http://www.richardelling.com/Home/scripts-and-programs-1/zilstat

ZFS Raidz
4 x 1TB Hard drives
8GB of DDR3 RAM
Primarily used for CFIS Shares and occasional FTP.


As Always Thanks for your help!!

 

[ProtoSD]

I think I remember one user here using an SSD for ZIL and it had some problem and he lost his whole array. It was either ZIL or Cache, if it's not mirrored and you lose it, you lose everything. Just keep that in mind.

 

[djlax152]

Sounds like that would have been the ZIL. I have taken my Cache disk out before and the Array was still working, i think i read somewhere that it will just not use the cache if the disk fails.. anyways YIKES !! If that is true then it isn't worth watever boost the ZIL would give me! I can't really afford to mirror the ZIL at the moment. Also it seems like if you had to mirror the ZIL you would loose some performance anyways. Thanks for the post! good looking out

 

[survive]

Hi djlax152,

With the version of ZFS that FreeNAS currently uses the loss of the ZIL is fatal to your pool, so you *must* have redundancy in your ZIL. That means you need a mirror of devices that you use for the ZIL.

Think of it this way....when you add a ZIL it gets "grafted" onto the existing pool just like any other vdev, and just like any other vdev, once it's added it can't be removed without dire results. Later versions of ZFS don't have this restriction \ drawback.

That said, are you sure you are actually even putting enough of a load on your filer to even need a ZIL? Are you seeing any sort of performance problem or is this being driven more by your gut than anything?

I see you have a DDR3-based system....if you really want to put some money into the system I'd look into swapping out \ adding more RAM.

Take the $100 that a cheap 30GB SSD would cost, get a pair of 8GB DIMMs and be done with it.

-Will

 

[djlax152]

Thanks for your reply Will, I sometimes i notice a lag mostly when i am browsing through my directories and launching files (i.e. read operations). This is on a windows PC through my CFIS share. I never had this problem when i was using a windows PC as a file-server. that's why i was thinking maybe I need a performance boost. Only problem i have is when i first started this project i was thinking that i was going to be leaving my Freenas on all the time so i tried to go for a low power consumption Motherboard and it only supports up to 8GB. I'm really regretting that decision, I should have researched this more before i went that route but oh well. I guess it looks like the next best option i have is to use SSD drive for cache although like you said I'd rather do the RAM upgrade but unfortunately i can't. Thanks for the Reply Will I really appreciate your insite!

Cheers!!

 

[louisk]

FWIW, typically you don't need a SSD for ZIL until you're well above 1G throughput. For example, you would want a fast ZIL for 10G. If you're not above 1G, I don't think you'll see any performance change from a faster ZIL. You might see a performance change from an SSD L2ARC, depending on whether you're getting the same data over and over or not.

 

[djlax152]

Great! thanks Louis I for sure don't have that kind of throughput. Good advice! Not sure if you know this or not but in trying to gauge weather or not my SSD is making a difference or not, what do you think in my case i will notice a difference. From what i have read I should within reason be experiencing better Read time when i'm browsing through my share especially for frequently used data. What do you think?

 

[louisk]

I notice that you have <8T of storage, so you should be fine with 8G of RAM (general rule is 1G RAM for each 1T of storage, not counting dedupe).

What kind of lag are you seeing that you think should be faster?

There is a relatively cheap way to test. You can buy a 32G SSD for something like $50 and add it to your pool as a L2ARC. You don't have to worry about removing it because cache drives can be removed on the fly, although it might be polite to power off the NAS and then remove it.

 

[monarchdodra]

Network

Unless you are putting any kind of real strain on your nas, I'd doubt throwing any kind of hardware at it would do anything at all, 8G ram is more than enough, and hard drives are fast, unless you are trying to retrieve thousands of little files. You describe it mostly as a media repository. Apart from serving you big files, it sounds like it isn't doing anything complicated.

I'd put my money on the network layer. That is definitely where I think the bottleneck would be:

Either the network itself. Chances are it can support a single user downloading a movie from the nas, but is too tight to support concurrent downloads. Use iperf to see the bandwidth you can get between your computer and your nas.

Either the CIFS protocol itself. Samba isn't exactly known for performance. Try playing around with the AIO, large packet options.

My 0.02$ before you spend your money on stuff you don't need.

 

[djlax152]

I notice that you have <8T of storage, so you should be fine with 8G of RAM (general rule is 1G RAM for each 1T of storage, not counting dedupe).

What kind of lag are you seeing that you think should be faster?

There is a relatively cheap way to test. You can buy a 32G SSD for something like $50 and add it to your pool as a L2ARC. You don't have to worry about removing it because cache drives can be removed on the fly, although it might be polite to power off the NAS and then remove it.

Thanks for the Tips!! that Rule of thumb should be in the WIKI.. Or maybe it is and i missed it! great tip!

 

[djlax152]

Unless you are putting any kind of real strain on your nas, I'd doubt throwing any kind of hardware at it would do anything at all, 8G ram is more than enough, and hard drives are fast, unless you are trying to retrieve thousands of little files. You describe it mostly as a media repository. Apart from serving you big files, it sounds like it isn't doing anything complicated.

I'd put my money on the network layer. That is definitely where I think the bottleneck would be:

Either the network itself. Chances are it can support a single user downloading a movie from the nas, but is too tight to support concurrent downloads. Use iperf to see the bandwidth you can get between your computer and your nas.

Either the CIFS protocol itself. Samba isn't exactly known for performance. Try playing around with the AIO, large packet options.

My 0.02$ before you spend your money on stuff you don't need.

Thanks, I originally was going to spend 100+ dollars on an SSD Drive but i think my problem was in the system Tunables. I installed my 8GB of RAM but Freenas wasn't utilizing all of it but i was able to fix it with some advice. the thread is here if anyone is interested.

http://forums.freenas.org/showthrea...M-to-8GB-Doesn-t-seem-to-be-utilizing-all-8GB

This really boosted my performance and like monarch said I'm not really stressing the system too much so i don't really feel like a need it anymore.

 

[madmax]

http://doc.freenas.org/index.php/Hardware_Recommendations

been reading this docs but I really don't have the answer I want for finding a SSD for ZIL and Cache device.

for ZIL log, I won't benefit from ssd so can I go with two Compact Flash or SDHC cards in USB for ZIL log? They will be in mirror and be the same type, does this work? Is this a good idea? What is a ideal size for the cards?

Not sure what this means from documentation

SSD cache devices only help if your working set is larger than system RAM, but small enough that a significant percentage of it will fit on the SSD.

What does it mean of working set, assume in my situation a video file being 30 GB. So any file that large then my RAM (16 GB) and file that smaller then my SSD (90 GB). So essentially when making a recommendation for size of your caching device larger then your ram and larger then the files that your working with or maybe transferring in my case or accessing. I don't work with more then 50 GB files or folders for transfer so I wouldn't need anything bigger. Is there SSD ideal for FreeNas Cache for reliability and performance? I guess in this case performance is more of an issue.

 

[louisk]

I'm a little confused about why you think you should put ZIL on a Compact Flash or SDHC card, or even why you think you need a dedicated ZIL device.

The rule I've used is that the data set needs to fit on the SSD in order to get a tangible benefit of using it. In this case, if you have a 30G video, you will need at least 30G of cache, and you will need to be accessing that file more than once (the first time it gets copied to the cache, the second time it can be read from the cache).

Cache will only help if you're reading/writing the same thing frequently, where it will be stored in the faster access of the cache. If you're not accessing very frequently, you won't see significant (if any) advantage because other data will replace it in the cache.