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"True Hot Swap" or just "Hot Swap"?

Western Digital Drives - The Preferred Drives of FreeNAS and TrueNAS CORE

Monkey_Demon

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Joined
Nov 11, 2016
Messages
81
According to Wikipedia:
There are two slightly differing meanings of the term hot swapping. It may refer only to the ability to add or remove hardware without powering down the system, while the system software may have to be notified by the user of the event in order to cope with it. ... This is sometimes called cold plugging. However, if the system can detect and respond to addition or removal of hardware, it is referred to as true hot plugging. Examples include USB, FireWire and higher-end SCSI devices.
Using appropriate SCSI drives with a motherboard & BIOS that support true hot plugging, this would imply FreeNAS does too. But this thread and this post suggest otherwise. For example, the post explicitly says,
"Shut down your server, remove the failed/failing disk, and replace it with a good, burned-in, tested disk. Power up the server."

I searched The Fine Manual for "hot swap" with no luck. So what's the deal? What, if any, kind of hot swapping does the current version of FreeNAS support?
 

blueether

Senior Member
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Aug 6, 2018
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259
it works well on old IBM server stuff that I have
 

Monkey_Demon

Member
Joined
Nov 11, 2016
Messages
81
it works well on old IBM server stuff that I have
Blue, please be more specific:
  1. Which kind of hot swapping works well? What Wikipedia calls "true hot plugging" for which you don't have to shut down the machine? Or what Wikipedia calls "cold plugging" for which you do?
  2. What old IBM server stuff? A 360 mainframe? A first-gen 8080 PC? A S922? The point here is that IBM has been making specialized servers for over 50 years, and many of them have specialized hardware one cannot ordinarily get on systems running FreeNAS.
 

danb35

Wizened Sage
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Aug 16, 2011
Messages
12,419
this post suggest otherwise.
My guide that you link to makes no assumptions about the hardware capabilities of the user's system, in particular whether and to what degree the hardware supports any form of hot swap, and therefore suggests nothing with respect to your question--it simply isn't intended to address it.

FreeBSD (on which FreeNAS is based) supports hot-swap disks just fine, assuming appropriate hardware (such as what I have, which is noted in my .sig). If I plug a disk into an empty bay, the system will see it and assign it a device ID (e.g, /dev/da17). What it will then do with that disk varies. If that disk is known to be part of an existing pool on the system, it may automatically online the disk and start resilvering. If it's a new disk, or a disk that isn't already part of an online pool, the system won't automatically do anything with it; you'll need to tell it what to do.
 

Monkey_Demon

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Joined
Nov 11, 2016
Messages
81
My guide that you link to makes no assumptions about the hardware capabilities of the user's system, in particular whether and to what degree the hardware supports any form of hot swap, and therefore suggests nothing with respect to your question--it simply isn't intended to address it.

FreeBSD (on which FreeNAS is based) supports hot-swap disks just fine, assuming appropriate hardware (such as what I have, which is noted in my .sig). If I plug a disk into an empty bay, the system will see it and assign it a device ID (e.g, /dev/da17). What it will then do with that disk varies. If that disk is known to be part of an existing pool on the system, it may automatically online the disk and start resilvering. If it's a new disk, or a disk that isn't already part of an online pool, the system won't automatically do anything with it; you'll need to tell it what to do.
Thanks for the explanation.

My bigger concern is removing a disk already being used. E.g., suppose someone wants to relocate a drive to a different slot or just remove it to read when it what manufactured. Assuming the system can handle hot swaps, will FreeNAS handle such examples gracefully?
 

danb35

Wizened Sage
Joined
Aug 16, 2011
Messages
12,419
Assuming the system can handle hot swaps, will FreeNAS handle such examples gracefully?
With recent versions, probably, though a better practice is to offline the disk through the GUI first. Older versions didn't mirror swap, so if swap was in use on that disk, removing it without offlining it first could be catastrophic.
 
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