Backup power of ZIL SSD

[Jan Rojek]

Hi,
Did any of You FreeNAS users ever thought about making an external power backup for consumer grade SSD used as ZIL SLOG?

I know that the best option is to buy capacitor enabled SSD, like Intel S3700.
But.. Wouldn't adding something like this:
http://robokits.co.in/raspberry-pi/battery-backup-system-for-raspberry-pi-and-other-5v-applications
between power supply and SSD drive make it power loos protected?

Crazy idea? Might work?

My setup is:
Dell C2100
2x Xeon X5570
50GB RAM
LSI SAS1068E IT mode
12x 600GB SAS 15k in RAID10
Samsung 850 Evo 250GB SSD for ZIL
Dual 10GBE Solarflare ethernet adapter

What I want:
Fast synchronous writes and high IOPS via iSCSI used by 3x ESXi hosts

What i know I should do:
Add more RAM - that will be done next month. Planning to make it 128GB.

Regards,
Jan

 

[Grewterd]

I think (and this is just my relatively low experience opinion, take it as it is) instead of experimenting with your data, buy a quality UPS and setup monitoring in FreeNAS so that it will shut the server down cleanly in the event of a power outage. Why worry about powering the ZIL if the rest of the system has gone dark?

From the best practices guides, your ZIL should be mirrored also.

I don't have enough experience to advise you on the rest save to say read every sticky on the hardware forum and go to every link in every sticky and read them. Then do it all again. I began to see repeating themes in all of them regarding how to setup FreeNAS best, what I should and should not do, what I needed and did not need, what works and what doesn't, what is trusted and proven and what is not.

Hope this helps!

 

[HoneyBadger]

In theory this might work but the SLOG device wouldn't have any knowledge of host power loss. I'm sure that a small inline battery would provide more than enough power for an SLOG device to flush any volatile cache to stable NAND, but the SSD might still try to perform its own housekeeping/TRIM/garbage collection and those could be interrupted in an unknown state when the power does die/SATA link reestablishes. For a device that's got its own "internal battery" it will know that the host had a power failure, and act accordingly (flush volatile cache to stable NAND, stop all other tasks, safely halt) to ensure the best chance of coming back up when host power is restored.

ZFS admins are generally a pretty risk-averse type; I mean, we are running ZFS, after all. I certainly wouldn't trust a system like this to hold critical data when there's a known stable alternative. A test box, somewhere I have good backups and won't lose my job/sanity if I have to roll back to a minute/hour/day ago? More likely, but in that situation why wouldn't I just run sync=standard?

What I want:
Fast synchronous writes and high IOPS via iSCSI used by 3x ESXi hosts

Get a ZeusRAM or an NVMe Intel DC series if the SATA versions aren't fast enough.

 

[depasseg]

Why worry about powering the ZIL if the rest of the system has gone dark

If you don't know, then you probably shouldn't be providing advice. There is a huge difference. This can seriously degrade your pool. And this isn't limited to a power outage. What about a crash or hard reset? The UPS is irrelevant in that case, and the SLOG cache will be wiped.

 

[jgreco]

I have to concur. The thing that makes a protected SSD special is that it is designed to be aware that the power is going away momentarily, and it takes appropriate steps. Merely making the SSD shut down at some random point after the host system shuts down can basically be arranged with a capacitor, but this doesn't add the ability for the SSD to understand the loss of power that's going to happen momentarily.

Don't halfass a SLOG. Either do without, and cope with the possible loss of data, or do it right, and be assured. Going halfway basically leaves you with the likely possibility that you haven't fully solved the problem and you're just back in the "possible loss of data" scenario, PLUS you're paying the performance penalty of having a SLOG.

 

[Jan Rojek]

Thanks guys, you are absolutely right about the fact, that the drive wouldn't know about power failure and might act unexpectible.
For now it is my test enviroment. I want to make something fast and as stable as can be, that will outperform Netapp FAS2240 not ruining company's budget. Before putting it to production I will replace Samsung SSD with a pair of mirrored Intel S3700. But.. now is a time to play and experiment :) Having a knowledge shared by all of You in this thread I can calm down and leave the idea of externaly powered SSD and do something less crazy :)

 

[jgreco]

Don't even necessarily need to worry about mirroring your SLOG, though having a standby device could be a good idea.

 

[Jan Rojek]

And what when the SSD dies during normal array operation? Wouldn't that lead me to a scenario in which I am begging all the saints to move me back in time and let me add a seccond device in a mirror?

Seriously, isn't having non mirrored SLOG a single point of failure? Is FreeNAS ready for unexpected desapearing of SLOG device?

 

[solarisguy]

Hi [B]Jan[/B], how did you arrive at having 50GB of RAM ?

 

[Jan Rojek]

You simply don't have to put same size memory modules. Mixing is possible with DDR3.
I have 6x8GB and 2x1GB.

 

[solarisguy]

You simply don't have to put same size memory modules. Mixing is possible with DDR3.
I have 6x8GB and 2x1GB.

Were you able to measure a difference in performance with 1GB modules and without them? Of course, only if 48GB RAM (as 6x8GB) is a valid configuration for this server.

I know that you did not ask for comments about your setup, but I would have hot-spares.

 

[Jan Rojek]

Modules are the same speed, so why waste it?
This server has 9 dimm slots per CPU, so your ammount of ram can be odd if using a single CPU.

Performance penalty? No, as it is supported.

Speed gain compared to 48? Don't realy think so :)

But if You have many modules in Your setup, every one of them is "something".

 

[jgreco]

And what when the SSD dies during normal array operation? Wouldn't that lead me to a scenario in which I am begging all the saints to move me back in time and let me add a seccond device in a mirror?

Seriously, isn't having non mirrored SLOG a single point of failure? Is FreeNAS ready for unexpected desapearing of SLOG device?

Yes, that's not really a problem except for the performance hit. That's why I suggested having a standby device available. If you mirror the two devices, then when one wears out, the other is probably nearly worn out.

 

[solarisguy]

[...] Performance penalty? No, as it is supported.

Speed gain compared to 48? Don't really think so :) [...]

I am expecting the system to be a little slower :D with 50GB of RAM. Supported ≠ faster.

Not too many people have similar configurations, so I was wondering whether you have performed any tests.