Not using SLOG

[wreedps]

Hello,

I have configured a logs drives to a Sandisk SSD but I have noticed my FreeNAS does not use it at all.

I use FreeNas as a Iscsi target for VMware. I see it using the Cache SSD.

What am I missing?

 

[depasseg]

The slog is only used for sync writes.

 

[wreedps]

When I enable the sync=always my performance tanks. I have a Sandisk x110 SSD I was trying to use for SLOG.
I use my Freenas as a Iscsi target for Vmware.

I thought enabling that would increase performance, but it appears to me it does not.

Maybe I am doing something wrong?

 

[depasseg]

When you enable sync=always, can you see the slog being used?

 

[wreedps]

Yes I did. I was slamming it with IOMeter so maybe that was the problem. Wasnt a "real world" experience/test

 

[HoneyBadger]

There's no* situation where sync writes will have better performance than async writes. Isn't going to happen.

An SLOG helps to lessen the performance hit - it doesn't eliminate it.

*maybe there's some theoretical situation but I sure as hell can't think of it

 

[HoneyBadger]

If performance is really bad, even considering the inherent loss, make sure you have your X110 configured as LOG and not accidentally striped into a CACHE vdev.

"zpool status poolname" in codeblocks please?

 

[wreedps]

Oh OK I was under the impression it would help significantly. It sounds like I shouldnt use a SLOG and just stick with faster drives and gobs of memory, huh?

 

[wreedps]

pool: SATAPool1
state: ONLINE
scan: scrub repaired 0 in 8h12m with 0 errors on Sun Nov 15 08:12:20 2015
config:

NAME STATE READ WRITE CKS UM
SATAPool1 ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
gptid/e68cd7fc-6e1d-11e5-809e-001b21a45f59 ONLINE 0 0 0
gptid/e7b077af-6e1d-11e5-809e-001b21a45f59 ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
gptid/e86d2754-6e1d-11e5-809e-001b21a45f59 ONLINE 0 0 0
gptid/e915b291-6e1d-11e5-809e-001b21a45f59 ONLINE 0 0 0
mirror-3 ONLINE 0 0 0
gptid/4a6d89d2-6e2a-11e5-8324-001b21a45f59 ONLINE 0 0 0
gptid/4c269349-6e2a-11e5-8324-001b21a45f59 ONLINE 0 0 0
logs
gptid/e9aa5aca-6e1d-11e5-809e-001b21a45f59 ONLINE 0 0 0
cache
gptid/e96bf276-6e1d-11e5-809e-001b21a45f59 ONLINE 0 0 0

 

[HoneyBadger]

Oh OK I was under the impression it would help significantly. It sounds like I shouldnt use a SLOG and just stick with faster drives and gobs of memory, huh?

SLOG has to do with safety of the data. Writing data, especially VM data, in asynchronous mode, means that the data is under risk if you lose power to the FreeNAS box during a write, as the writes are being cached in RAM only. If that was important VMFS metadata, worst case scenario is that you corrupt a VMDK or the entire datastore/filesystem and have to restore from a backup. (Which you have, right?)

If you want fast SLOG you should use something like an NVMe SSD (Intel P3700) or a ZeusRAM device (if you have money to burn)

Your pool is setup up correctly though.

 

[wreedps]

Hey thanks HoneyBadger, as always you provide great information!

Yes, I back everything up to a separate NAS with Veeam.

We have lots of those P3700s at work, I will see about getting me one to play with. I just want my FreeNAS at home to be incredibly fast, but at the same time I want it configured for safety too. I do not want to corrupt my VMs.
Luckily my servers(freeNAS) at home are on extremely large UPSes that are then tied to a GenSet.

 

[HoneyBadger]

With large UPSes and a generator backing them you would probably be safe, assuming you monitor UPS charge/health and change batteries as necessary. You could likely get away without SLOG.

But if your job is going to give you a thousand-dollar NVMe SSD to play with ... then by all means, take advantage of it. :D

 

[wreedps]

The batteries are replaced the minute the red light comes on. But enough about how I maintain my home network.

Does FreeNAS support FusionIO cards? I have stacks of those.

 

[wreedps]

Little eye candy for you guys.

 

[HoneyBadger]

The batteries are replaced the minute the red light comes on. But enough about how I maintain my home network.

Does FreeNAS support FusionIO cards? I have stacks of those.

It's officially supported in TrueNAS but I don't believe FreeNAS has native support. The last thread discussing it is here:

https://forums.freenas.org/index.php?threads/fusionio-driver.24514/

You can probably build the driver support into it, but it would risk breaking on update.

And okay, now I'm jealous. I wish my job gave me stacks of flash like that.

 

[depasseg]

We'll help you get the fusion io cards working.

 

[HoneyBadger]

We'll help you get the fusion io cards working. :)

Consider me starred on this issue. I haven't the time to futz with it myself, but if there's a turnkey solution that would result in "hey, FusionIO cards work now" I have an interest in that.

 

[wreedps]

Now just to rub it in :) 2 of 4 IBM SVCs

 

[wreedps]

I will try to get my hands on one. I have 4 or 5 FreeNas boxes we could play with. Hell if your nice, I will even give you remote access :)

 

[wreedps]

And Big Momma... This is how we sleep at night.