Adding SSD cache drive LARC2?

[katit]

Hello! I just got FreeNAS installed, used USB stick for OS. Before this computer was booting from 128Gb SSD. So, now I have this SSD laying around. In Wikipedia on ZFS I read about caching.
https://en.wikipedia.org/wiki/ZFS#ZFS_cache:_ARC_.28L1.29.2C_L2ARC.2C_ZIL

So, level 1 is my RAM (8GB). I thought it would be cool to put this SSD to use. Is it possible with FreeNAS to do what's described? I could use this SSD drive for that. I didn't see anything that relates in GUI options. Is it even implemented?

 

[Bidule0hm]

Yes it's possible BUT you don't have enough RAM to do it.

Please read Cyberjosk's guide (link is in my signature) ;)

Edit: edited because I posted garbage reasons to not use it (it's what happens when I read too fast at 3 am...).

 

[katit]

Cyberjosk's guide was very very helpful. Really helped me with some of the doubts. ZIL is for sync writes, LARC2 is for caching often used files, but from what I see - yes, I'm not even close to "needing to consider it". Will go with KISS principle.

My system is not that "great", non-ECC RAM. But it's OK, now I understand potential issues and I think I will be OK as I'm backing up anyway. And one of the backups is Crashplan with versioning, should help with corruption scenario.

I have about 500Gb of data, only 100 or so of them is critical..

 

[Bidule0hm]

Yeah, sorry, I mis-readed your post and answered for ZIL instead of L2ARC, I edited to correct my post ;)

Hopefully it's still true you shouldn't use a L2ARC.

Keep in mind that backup is useless for non-ECC RAM corruption if you backup from the NAS because you'll backup corrupted data... But if you backup from the original source it's ok ;)

 

[katit]

Do I understand correctly that "corrupted" will apply to files? If so, I think it is "OK" because crashplan has file versioning built in. If I detect corruption, for example, I can always check on that file and pull from a history

 

[Robert Trevellyan]

If I detect corruption, for example, I can always check on that file and pull from a history

The big problem with this scenario is undetected corruption. If bit rot occurs in one of your most precious JPGs, you won't know until you open that file and notice that it's broken. Then you will have no idea how far to go back in your CrashPlan history to find a clean version, assuming it even exists there.

We use ZFS because it detects corruption. Scrubs of suitably configured pools find corruption and fix it before it makes it into your backups.

We use ECC RAM for basically the same reason. This isn't an invitation to debate ECC vs non-ECC yet again It is an invitation for you to verify your risk tolerance.

Having CrashPlan (or any) backups can enable recovery from detected corruption, but they're much less useful for undetected corruption. I can vouch for this first hand. I lost a music file to bad sectors on a hard drive in a Mac mini. No matter how far back I went in the CrashPlan backups for that machine, I couldn't extract a good version. Then I found the original CD was no longer readable, so I couldn't re-rip it either. Not a disaster, I could buy the track again, but it illustrates the problem.

 

[katit]

Agree. Well, it is what it is for now. Nothing super critical in archives, other stuff is in many places, will take chances for now.

 

[Bidule0hm]

Remember too that non-ECC RAM corruption can destroy the pool metadata and you can lose the whole pool, it's not that likely to have a RAM error (with tested RAM) but I prefer to warn before than after ;)

 

[katit]

I realize that. For what I do - it's OK. I have 20Gb of really important data and it's stored in 3-4 places I think :) This is perfectly good reliable hardware I've used for 3 years with Windows file server. Now with my setup I'm getting about 85-80Mbps speeds on my network. I didn't check if it was SSD speed on receiving end or it's a speed of RAID1 on FreeNAS (2x 1TB WD Blacks in a mirror), more than acceptable. At home we don't put much load on a system, it's 1.2 user system :)

I read a ton about RAM issue and found some info that other file systems can suffer same failure but for some reason ZFS is mentioned more. Also, I found some info where ZFS actually helped with restoring data corrupted by bad memory. All in all, to get "reliable" NAS it will cost $2k up (the way I want it in a rack) and I really don't need much space, so I'm going with compromise.. Old-ish data backed up to many places as well..

 

[Robert Trevellyan]

I read a ton about RAM issue and found some info that other file systems can suffer same failure but for some reason ZFS is mentioned more. Also, I found some info where ZFS actually helped with restoring data corrupted by bad memory.

There's some disagreement over whether ZFS without ECC is worse than [any other filesystem] without ECC, but it's clear that [any filesystem X] with ECC is better than [any filesystem X] without ECC.

Clearly you're making an informed decision, which is what I care about.

 

[katit]

Main top question remains :) System boots up from Sandisk 64Gb, that was my mistake, I didn't have smaller thumbs nearby at the moment. Now, in order to create mirrored boot thumb I need to get another expensive thumb.
Or... How hard is that to reinstall system only? If I don't touch hdds, backup my settings and than reinstall fresh and restore settings, will it work ok?

And MAIN question: Is there any use to 128Gb SSD I had in this system before?

 

[danb35]

Reinstallation is easy--save the config file, do a clean installation (to both USB devices if desired; the installer can set up a mirror with unequal-size devices), boot the server, then upload the saved config.

I guess you could use the SSD to create a second pool and put your jails and such (maybe the .system dataset, if you really want your drives to be able to spin down) on that.

 

[Wim Mistiaen]

will a restore like that also restore the pool configuration? (i.e., will i see my raid-z pool back with all the data intact).

By the sound of it, it probably does, but I'm just askingto make sure. :)

 

[Bidule0hm]

Yes, no problem ;)

 

[katit]

I confirm it does. I shutdown system, removed old usb, inserted 2 Sandisks, reinstalled FreeNAS, upgraded and restored config. All volumes, shares, etc restored to how it was. Plugins, everything. Now it shows mirrored boot drive.