Mirror boot pool still recommended?

ragametal

Contributor
Joined
May 4, 2021
Messages
187
As the title says, I’m trying to get the opinion of the community with regards to mirror the boot pool.

I understand the intent and I like to principle behind it. Nowadays the investment of having an additional SSD for a boot pool is minimal.

I understand is not much trouble to replace a failed boot drive and restore a system if a proper backup has been performed. But with a mirror, I wouldn’t have to take the system offline at all until I get a replacement drive and that by itself is more valuable than the price of an extra SSD.

However, I read somewhere that this arrangement doesn’t work like most people think it does. When a drive is about to fail, it produces multiple I/O errors which will then expand to the mirror drive. In this scenario the entire pool would be corrupted and the administrator would have to restore the system from the backup anyways (just as if the system had a single drive).

I honestly never had an SSD die on me so I don’t know if they die like a fuse (they are either good or bad, nothing in between) or if they produce errors on their way out. If the latter is true, then I see little value in mirroring the boot pool.

Opinions?
 

danb35

Hall of Famer
Joined
Aug 16, 2011
Messages
15,456
No, I/O errors on the one drive wouldn't somehow spread to the other device. Eventually the failing/failed device would drop offline, and the system would continue with the other. The issue is what happens if/when you try to reboot. Depending on the exact way in which the device failed, it could result in your system trying to boot from it and failing.
 

Redcoat

MVP
Joined
Feb 18, 2014
Messages
2,924
No, I/O errors on the one drive wouldn't somehow spread to the other device. Eventually the failing/failed device would drop offline, and the system would continue with the other. The issue is what happens if/when you try to reboot. Depending on the exact way in which the device failed, it could result in your system trying to boot from it and failing.
To complement @danb35 's words, unless your BIOS has some magic failover mechanism, you'll have to there select your working mirror drive to boot from when your active drive has failed.
However, I read somewhere that this arrangement doesn’t work like most people think it does.
This smells a lot like spreading FUD. If you must make a statement like this, please identify the "somewhere" with a link in order that it can be appropriately critiqued or commented upon in context.
 

danb35

Hall of Famer
Joined
Aug 16, 2011
Messages
15,456
unless your BIOS has some magic failover mechanism
...or the device just completely fails. Let's suppose you've got two SSDs on SATA0 and SATA1, and the first one fails. If it's a hard failure--the device is just completely unresponsive--the system sees no device there, and looks to SATA1. And since that one's up and running, the system boots, and you're all good. The problem is if the first device, like the Man in Black, is only mostly dead. In the that case, it could hang or otherwise disrupt the boot process.
 

Evertb1

Guru
Joined
May 31, 2016
Messages
700
Personaly, since I dumped USB sticks as boot device in favor of a single SSD I am happy with my boot device. I try to keep my system as simple as possible. My SSD is running for 4 years now, something I could not say about those USB sticks. Reinstalling and uploading your config is a quick job as you already mentioned. But I don't mind some down time of the server when needed. And I always have a couple of small SSD's lying around. My file server is mainly for home use and for support of my work when I am working from home. Besides the unavoidable down time when the boot device fails (not happened with the SSD yet) I don't see any special disadvantage of running my system this way.
 

danb35

Hall of Famer
Joined
Aug 16, 2011
Messages
15,456
I have had a SSD fail in my then-FreeNAS server, but it was storing jails, not acting as the boot device. There's another SSD in there as the boot device, it's been there for five years or so, with no problems.

So on the one hand, there's no real reason not to use mirrored SSDs for boot. And there will be some increase in reliability--the system will keep running in the event of a device failure, even if it might not boot. But the reliability of SSDs, and the generally-low utilization of the boot pool, combine to result in a generally-long life for those, so there's not a super-strong reason to do it, either.
 

ragametal

Contributor
Joined
May 4, 2021
Messages
187
Thank you all for the responses.

Now I understand what the issue is and I can take an educated decision on whether or not to mirror the boot pool. Based on your experiences it appears that a typical boot SSD should have a life span of at least 5 years (most likely longer). At that point, I'm OK with a single drive.

Let's face it, I will take the system down before that anyway for one reason or another (to replace UPS batteries for example). If my boot drive fails, it shouldn't take me more than 2 hours to buy a new one at best buy and restore the system from the backups.

@Redcoat I tried to find the source where i read about the I/O errors but, and i think this is the point you wanted to make, i could not find it. I've been reading so many forum post lately that everything is becoming a blur. I'll keep this in mind for future post as the last thing i want is to spread innacurate information on the forum.
 
Top