Newbie question about mirrored SSD for SLOG

[Vanarie]

I've never setup FreeNAS, but I'm getting there.

I setup a 6 x 2T disk system, 8G ECC ram and one MLC SSD for FreeNAS OS and SLOG. I started reading that SLC SSD is recommended. Because I'm paranoid about data integrity, I'm wondering if I can run this with using 2 x MLC SSD mirrored to avoid possible SLOG corruption? This is a home storage solution, so there's not going to be a lot of I/O as compared to a business system.

And if so, I assume I would use the MB bios to setup this OS mirrored drive. I'm running with a SUPERMICRO MBD-X9SCL-F-O.

 

[cyberjock]

You seem to have gotten in over your head..


Unless you think you need an SLOG, you don't. And for workloads where an SLOG is beneficial, you're going to need more than 5x your RAM anyway. And, you should NOT be using the same device for your boot device and SLOG (not that you could do that without some OS hacking).

So I'm really confused at why you are asking the questions you are. Your thought process on what hardware to use and how to configure FreeNAS seem to be at odds with each other (and at odds with how FreeNAS and ZFS is designed).

 

[Vanarie]

I'm so sorry, I totally messed up my terms here. I meant ZIL.

From the FreeNAS Guide 9.2.1 from Cyberjock (looks like this is non other than you)...

• ZILs should have their own redundancy from drive failure in environments where
  zpools need redundancy. It is highly recommended that ZIL drives be operated in a
  mirrored mode to prevent data loss for this reason(even after v19). Otherwise a failure
  of the ZIL will result in a loss of all data that is in the ZIL and not committed to the
  zpool. A loss of data in the ZIL can, in certain circumstances, result in data corruption
  due to uncommitted data being lost. This may cause your entire zpool to be lost.
  ZIL is only useful for sync writes. For all other writes the ZIL is not used. This means if
  you aren’t using NFS and don’t have a workload that uses sync writes, a ZIL is pointless.
  For 99.9% of home users, a ZIL will not be useful.

So I can assume then that I don't even need ZIL (disabled in the config)? I just want to set this up so it's as stable as possible (within reason).

I've read you can just run the OS off of a thumbdrive, but I'd rather run it off of an SSD since there's less chance of removing the thumbdrive by accident, etc. Just SEEMS more stable that way.

Thanks for your help.

 

[gpsguy]

You'll still have a ZIL. What you don't need is a SLOG. See jgreco's explanation of the two topics in this thread - https://forums.freenas.org/index.ph...log-zil-with-zfs-on-freenas.13633/#post-81457

Historically, most users have run FreeNAS off a flash drive, since the OS used UFS and the image size was ~2GB. If you had a 60GB SSD, you could only utilize ~2GB of it.

When 9.3 was released, support for UFS was dropped. The OS now runs on ZFS and it allows you to utilize the space on the drive. And, since it uses ZFS, you have the ability to mirror the boot devices. Some users are using mirrored flash drives, while some of the cool kids use SATA DOM's. Of course you could use a SSD too.

BTW, you don't use the mobo to create the mirror (fake raid). You need to use FreeNAS.

 

[Vanarie]

And thanks for the answer about not using the mobo raid. I'll set it up within the FN GUI.

Part of my question was about running 2 mirrored SSD drives that are MLC (instead of SLC) for more data integrity and less chance of ZIL corruption. It's not THAT expensive to get two smaller SSDs these days, so I'm not worried about the expense. Just want to build a rig that is reliable, even if it might be a bit overkill. The way I see it, data integrity is more important in the long run than saving $100 to build it.

 

[gpsguy]

No, USB boot is still supported. In fact, I said that some folks are now using mirrored flash drives with 9.3.

Support for UFS (a file system) was dropped.

 

[Vanarie]

My last comment was fixed to avoid confusion. Thanks. I guess I can assume that mirrored MLC SSD drives will work OK for this. Just wanted to check and see if it was a good idea. I down know if dual flash drives is equal to dual SSD drives. Probably an issue of hardware failure that can occur at any time in any drive system (flash, ssd, disk).

 

[cyberjock]

My last comment was fixed to avoid confusion. Thanks. I guess I can assume that mirrored MLC SSD drives will work OK for this. Just wanted to check and see if it was a good idea. I down know if dual flash drives is equal to dual SSD drives. Probably an issue of hardware failure that can occur at any time in any drive system (flash, ssd, disk).

Yes, but again.. as I said at the beginning, you probably don't need it. I understand you saying that you "can" do it to make it more reliable. You are wrong with this assumption. Adding more parts (even if those parts has redundancy) also means higher chance of problems. So unless you need the ZIL/slog device, you should NOT be adding them. Fewer moving parts means fewer things that can fail you.

So unless you *know* you need the ZIL/slog device, you should not be adding it. Doing so won't help you and will only cost you money that could be spent elsewhere and increase your risk of future problems. Also, as I said above, you're contradicting yourself by gong with this whole idea of a ZIL/slog device but then only having 8GB of RAM. You're going to be WAY more pissed with your small ARC size than your ZIL/slog. If you're ZIL/slog is *actually* necessary for you, then you're going to find out after you add it you still need more RAM than your system can support.

This isn't Windows. Throwing more hardware at a problem (or even a perceived problem) is NOT always the best way to go.

 

[Vanarie]

So, when it comes down to it, booting from a 8 or 16 gig flash drive should be fine in this case. If ARC usually eats up about 85% of RAM, then I should be looking into more RAM instead of worrying about enabling the ZIL. (hopefully I got that right)

Thanks, still learning! I really appreciate the quick response and your hard with FreeNAS.

 

[gpsguy]

Correct! Buy quality flash drives - like Sandisk, ... - not something you'd find at Big Lots.

So, when it comes down to it, booting from a 8 or 16 gig flash drive should be fine in this case.

When buying RAM, get 8GB sticks. Down the road, if/when you need to max out the board with 32GB, you won't be have to put the (old) smaller ones on the shelf.

 

[Vanarie]

I'll start with 16G RAM and forget about enabling ZIL. Thanks all.

 

[jgreco]

instead of worrying about enabling the ZIL. (hopefully I got that right)

The ZIL is always there and isn't something that needs to be "enabled". In an average ZFS system, it is mostly used for metadata updates and so the in-pool ZIL is perfectly adequate to the task. It is mostly something that needs to be worried about in a heavy sync write environment, database, VM storage, etc.

SLOG devices are something that can be added if your performance is suffering due to the use of in-pool ZIL.