What is the difference between a hot-swap drive and a standard drive?

[Tim Malnati]

In another thread there are some warnings regarding adding a non hot-swap disk to a live system. What I'm trying to get to here is a more detailed explanation of what allows a hot-swap unit to not create the same sort of problems. Is there a short delay between plug-in and power-up? Why doesn't the new device cause a bus reset? SCSI termination is not impacted when adding the hot-swap unit, but this could be accomplished with a standard drive by connecting a mid bus connector, so I sort of doubt that termination is the only impact.

[Shannon Petry]

There is only one major difference between a "Hot Swap" driver and not and that is the ability to be removed and added without screwing up the drive.
It is my understanding that: Normal drives require spin time, bus checking, etc...A hot swap drive has very little warm up time (spin time) to be able to read/write as well as power management to alleviate spiking on an active bus.

You can still have the same problems with both, depending on how they are used.
Normally, hot swap drives are in RAID configurations, so it makes sense to be able to move disks in and out with out downing the drive/system. I have seen systems which claim to have hot swapable drives, but no RAID. This means that you can't swap unless the file system can be unmounted. And of course without RAID there is no recovery method other than standard backup and restore....

I have especially seen "Hot swapable drive bays" in PC's.

Logic is such a drag sometimes eh?

Some people buy anything that sounds high-tech :)

Regards,
Shannon

[rick jones · Accepted solution]

It is _very_ important to keep clear the distinction between "hot-swap" and "hot-plug." Hot-swap is a superset of hot-plug.

A hot-plug disc is generally setup so that when it is plugged-in ground is the first thing to be connected. It is the last to disconnect when unplugged. That keeps the electronics from possibly being fried.

With a hot-plug disc, other commands have to be executed to break the mechanism out of say a mirror. Commands have to be executed to have a hot-plug disc used when added.

Hot-swap is hot-plug, but with all the command stuff happening automagically for you. This is generally with RAID arrays.

Many HP 9000's offer internal _hot-plug_ mechanisms, but they are not _hot-swap_. Also, "RAID-free" external enclosures like the SC10 are "hot-plug (iirc). The hot-swap is on things like the AutoRAIDs, FC60's etc...

[Daniel Malcor]

An HP Tech visiting out site recently told me that the Jamaica (A3312A) drive enclosure is "Hot-Plugable" and not "Hot-Swapable".
From Rick's description above that would be an acceptable distinction. But the point the Tech was making was that we should not replace the bad side of a mirror while the Server was live (even after the proper LVM commands were run). If this is true, then why buy a Jamaica?

[Tim Malnati]

I'm not sure where your tech is coming from on this. Rick's explanation seems to be sound. I've built a fair number of servers using Jamaicas for vg00 and as part of the initial testing sequence I pull the primary disk(s) while it's running, see if I get the appropriate messages in syslog, and then make sure that the system properly boots from the mirror set. I've also had some occasions to replace a bad Jamaica and not shut down the system to do so. It would probably be best for all reading this thread if you could get back to that tech to find out specifically where he is getting his information from (a document) and post the location of the source here.