Slog? Do I *really* need it.

[kspare]

Ok, so first things first...I made a mistake. Not sure how because it's not how I planned to set it up, but eitherway, attention to detail can kill you sometimes!

I had intended to made a bunch of mirrored vdevs. I wanted to have two mirrored ssds as my slog. However they ended up included with my main storage.

So this whole time i've been running with no slog and a single ssd as my l2arc.

So now I have a single drive as my slog and two drives striped for my l2arc.

We've now noticed a drop in performance. I've been told these intel 530 ssd's won't be the best and they aren't.

SO in the interest of starting a conversation about this, how safe is it really to run without an slog and when do you really want to put one in?

There is the option of adding in a pcie intel 750 when I can get my hands on one..but really... do I need one?

We have UPS power for 1 hour and in about 4 weeks we'll have a natural gas cummins generator. So.... do I really need an slow?

We host a ton of terminal servers on vmware iscsi, so l2arc seems to be helping there while we only have 64gb of ram...just not sure about the slog!

Thoughts?

 

[Mlovelace]

You should be setting sync=always for your iscsi volumes servicing VMware guests. So if you are following that rule then an slog device should help write performance. Just make sure the device for the slog is power loss protected, regardless of your ups system.

 

[HoneyBadger]

Hey, I remember that thread.

If performance has dropped after adding an slog, then you either didn't have sync enabled in the first place or the 530 is even worse than I thought.

How safe is it to run without SLOG?
Depends on your data. For VMFS, pretty dangerous because there's no way to determine what data is precious and what isn't, so you've got to treat it all as critical, or accept the risk of potentially hosing the entire datastore if you have a crash on your storage. About the only situation where it might be an "acceptable risk" is for something like a VMware View floating linked clone pool.

When do you want an SLOG?
When the data's critical - see above - because performance sucks without one.

We have UPS power for an hour, and generator soon.
Dual redundant PSUs in the server would definitely be absolutely required, but you'd still be running a risk. Having an HBA or backplane glitch out on you could still result in a wonky pool state if it was async. With sync, you'd at least have those writes in the SLOG so when you pieced it back together, it would play back properly. Although an HBA/backplane failure is pretty rare in my experience.

But if you can coax the budget out, go for a P3700. ;) A ZeusRAM would be best, but that's serious money.

 

[kspare]

Where do you exactly turn on the sync=always for iscsi?

 

[kspare]

I was thinking of two Intel PCIe 750s, one for slog and one for l2arc.

 

[HoneyBadger]

Where do you exactly turn on the sync=always for iscsi?

You'll have to do it from the command line for each zvol:

Code:

zfs set sync=always zpoolname/zvolname

I was thinking of two Intel PCIe 750s, one for slog and one for l2arc.

A 750 is definitely overkill for the L2ARC. L2ARC just requires fast random reads, which any modern SSD such as the 530s can handle. But you can always refer to my signature for my feelings on overkill. ;)

 

[kspare]

if I was able to take out the 3 ssds I could run 24 drives in 12 mirrored vdevs...just more performance!

What would the command be to view current settings?

 

[HoneyBadger]

"zfs get variable poolname"

"zfs get all" also works and you can then grep out the variable you want if you have multiple datasets.

 

[kspare]

Ok, so my sync has been set to standard. for the sake of making it easy for others to figure this out, what is the difference between standard and always?

 

[kspare]

I set sync to always. I have one Intel 530 SSD as the ZIL. the system isn't usable at this point. I'd have to go back to standard for now until I can get a much faster drive to act as my slog.

 

[HoneyBadger]

Standard won't write sync unless asked. VMware doesn't by default over iSCSI.

Always, well, always writes sync.

Here's a post from @jgreco on the subject:

https://forums.freenas.org/index.ph...xi-nfs-so-slow-and-why-is-iscsi-faster.12506/

 

[kspare]

Well, until I can get a better slog device I don't have much of a choice. I'm just going to run with my 3 ssds as l2arcs and no slog. our san has redundant power supplies, we haev ample power to shutdown in the event of a powerloss. short of that until I get a better slog device I think thats the best I can do.

I know it puts me at some risk if the server crashes, but we do also use vmware replication every 6 hours to another san. so that helps mitigate some of the risk I guess.

 

[jgreco]

A 750 is definitely overkill for the L2ARC. L2ARC just requires fast random reads, which any modern SSD such as the 530s can handle. But you can always refer to my signature for my feelings on overkill. ;)

Oh, I don't know. I haven't been impressed by delivered read speeds from L2ARC on a Samsung XP941 M.2 (AHCI) SSD. Overall it is very fast, managing 1GByte/sec reads, but by the time you get that delivered to a client, it was a less impressive ~200-250MB/sec, a single VM trying to read data randomly from it (and ARC was probably contributing a bit to that). My interpretation of that was that it was still struggling with latency a bit.

With the low price of the 750, I am definitely tempted to order one and see how well that does for L2ARC.

 

[kspare]

Interesting read here:
http://www.anandtech.com/show/8104/...ew-the-pcie-ssd-transition-begins-with-nvme/3

Intel 910 SSD sounds like it should be a real contender?

We have 2 sans, on is the primary for file servers and terminal servers the second is for non prioirty vms and backups. I'm thinking a P3700 400gb for my primary as an slog and turn on sync always and a Pci 750 400gb for my secondary with sync always on.

should be an interesting balance?

 

[jgreco]

If you do, see if you can get some performance comparisons between the 750 and the P3700.

 

[kspare]

Will do. I've migrated my data off of this box, i'm going to do some more testing. These intel 530's don't seem to be all that great for what i'm trying to do, the same goes for l2arc, my 6GB/s disks are hitting fast read speeds lol

 

[kspare]

Ok so to set the record straight and correct me if i'm wrong.

If you are using iscsi with vmware, and you don't set sync to always, an SLOG is pointless?

 

[Mlovelace]

If you are using iscsi with vmware, and you don't set sync to always, an SLOG is pointless?

Correct

 

[kspare]

ok perfect. I guess i'll run with 3 intel 530s in a stripe for l2arc until my intel 750 shows up!

 

[GangstaRIB]

ok perfect. I guess i'll run with 3 intel 530s in a stripe for l2arc until my intel 750 shows up!

Sorry to revive an old thread. I'm in the beginning stages of my build and I have a 750 showing up soon. Hows the SLOG performance? I'm still not sure If I want to open it... 400GB SSD ... ~$300 but SLOG will use 10GB if I'm lucky.... I wonder what the sync performance is. I will be running iscsi but I want to use sync write even though at the end of the day it's for a lab, but my applications are very DB intensive. I have heard horror stories from many friends about losing their labs due to power outage which is why I'm picking FreeNAS to begin with over the other solutions.