You can also do a "show boot-preference" to see which slot your unit boots though it doesn't tell you the image.
For example, on a stack of 7450-48F units (similar to your units):
Boot system preference(Configured):
Boot system flash secondary
Boot system preference(Default):
Boot system flash primary
Boot system flash secondary
Also if you do a "show run" I see this:
boot sys fl sec
The difference to make the boot option permanent is to do it in global configuration.
Switch>enable
user:
pass:
Switch# conf t
Switch(Config)# boot system flash secondary
Switch(Config)# exit
Switch# wr mem
You may want to know what is available:
Switch#show flash
Stack unit 1:
Compressed Pri Code size = 29823132, Version:08.0.80cT213 (SPR08080ca.bin)
Compressed Sec Code size = 29819448, Version:08.0.80dT213 (SPR08080d.bin)
Compressed Boot-Monitor Image size = 786944, Version:10.1.14T215
Code Flash Free Space = 1749340160
The above SPR makes it Layer-3 "Routing" code. SPS is Layer-2 or "Switch" code.
If I were to change this device to boot Primary, it would boot SPR08080ca.bin
That said,
By convention for Ruckus ICX, Brocade, Foundry, and other FastIron based switches, it is by convention when running Layer-3 code we boot from the secondary. They ship new in box with Switch code in the primary slot and Routing Code in the Secondary slot (if you buy a layer-3 capable device).
In the case of mine above, you will notice that I have routing code in both the primary and secondary slots... that Primary is simply an older build. This is because when I upgrade the firmware, I backup the existing code to other slot. In this case when I upgraded SPR code from 08.0.80ca to 08.0.80d, I ran the command "copy flash flash primary." Once verified complete I used "verif md5 pri" to validate the MD5 checksum to ensure the backup image is intact in the unlikely event it is needed! At that point, I upgraded the bootrom and copied SPR for 08.0.80d to Secondary. I also checked the checksum on the new secondary.
If I was updating switch code such as SPS code, I would by convention be booting from Primary (I leave it set to default) for this. If I was doing a backup, I would copy Primary => Secondary to back switch code up.
The reasoning is that if there is a problem, with a console cable you can get to the boot-monitor and booth whichever slot you want. In the event something goes wrong and the device fails to boot up, it would almost certainly boot to boot-monitor. Otherwise, it would be in a boot-loop that could be interrupted by pressing "b" when prompted.
Hope that helps. I know it is a lot of info, but it is the why, the how, and the reasoning.
*knocks on wood* I have never had an ICX device fail to boot after a firmware upgrade!