Is it possible to combine boot disk with SLOG?

[candlerb]

Hello,

I'm looking to build a small FreeNAS server (8GB RAM) doing some lightweight VM image storing at home.

I have a 120GB crucial SATA SSD. I can use this to boot from, but it seems a bit of a waste just to use it for this. Is it possible to partition so that part is boot area and part is SLOG?

I did google for this, but couldn't find anything. The installer just wants to take over the whole boot disk.

Thanks,

Brian.

 

[dlavigne]

No.

 

[danb35]

As @dlavigne says, no. And even if you could, that isn't a very suitable SLOG device.

 

[Ericloewe]

Vaguely possible? Yes. Good idea? No. Especially not on a consumer-grade SATA SSD.

 

[candlerb]

> Especially not on a consumer-grade SATA SSD

Can you elaborate on that? Is that on reliability or performance grounds?

Reliability-wise, the worst I can see happening is that one transaction group's worth of (synchronous-write) data would be lost, if the SSD failed at the same time as the power cord was pulled. That sounds like a risk I can accept.

Performance-wise, I'd have thought that any SSD would be better than writing the ZIL to the hard drives and then writing the transaction group a second time to the same hard drives, potentially while concurrent read accesses are going on.

The SSD I have to hand is Crucial m550 128GB, which according to this can do 350MB/s sequential write and 75K random IOPS. I could also enlarge the HPA for endurance, and still have plenty of space.

I realise that for maximum performance I could just set "sync=disabled", but then I have to accept a *really* high chance of data loss if the power is pulled.

So is there really no point at all in having a regular SSD as SLOG?

I could also boot from USB flash and use the SSD as dedicated SLOG - but given my experiences with USB flash sticks I'm not inclined to see that as a reliable solution either.

 

[Dice]

Can you elaborate on that?

https://forums.freenas.org/index.php?resources/hardware-recommendations-guide.12/
https://forums.freenas.org/index.php?threads/some-insights-into-slog-zil-with-zfs-on-freenas.13633/

So is there really no point at all in having a regular SSD as SLOG?

Correct. From a data security point of view.
When the pool is on HDDs ex Raidz2 thus io capped, there is a performance argument for using a consumer grade SSD for speed.
As you realize, at that point it makes more sense to use sync=disabled.

 

[Ericloewe]

Is that on reliability or performance grounds?

Both.

Reliability-wise, the worst I can see happening is that one transaction group's worth of (synchronous-write) data would be lost, if the SSD failed at the same time as the power cord was pulled. That sounds like a risk I can accept.

If you're going to take the loss of a TXG or two with a "meh", just turn off sync. Same result, better performance - especially since the SSD has no power loss protection and is pretty much guaranteed to fail in some way when the cord is pulled.

The SSD I have to hand is Crucial m550 128GB,

Same problem, no power loss protection.