kernel on usb stick

aych · Guru Joined: 20 May 2004 Posts: 304

Just wondering if anyone has any insight onto the effect of placing the kernel on a usb key? would I experience an increase or decrease in bootimes in that case? It probably reads slightly slower than a hard drive but there is no seek time.

Sadako · Posted: Sun Jul 12, 2009 11:35 pm Post subject:

I presume you mean the entire contents of /boot, not just the kernel?

Once the kernel actually loads, it wouldn't slow down the boot process at all, but it wouldn't make it the least bit faster either; the kernel is read just once at boot and loaded into ram.
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aych · Guru Joined: 20 May 2004 Posts: 304

yes, I meant the entire contents of /boot. I am planning to do a software raid 0

frostschutz · Advocate Joined: 22 Feb 2005 Posts: 2977 Location: Germany

Loading the kernel from the USB key will be slow. Because while most boards support boot from USB these days, they tend to do it in the (simpler) USB 1 mode, at a very low transfer rate <1MB/s. So if your kernel is 5MB because you compiled in a lot of drivers and added an initramfs, loading the kernel image alone will take over 5 seconds.

Once it's loaded the boot process will continue normally. If your root filesystem was actually stored on USB as well, it would again slow down since the Linux kernel takes some time to detect USB devices, so you'd have to wait another couple of seconds until you can actually mount the root partition.

So you will lose between 5-10 seconds when booting from USB, compared to booting from a faster media.

If you boot often this may become a nuisance. However if you're the type who boots once a day and then just lets the machine run, there'll be no difference at all, since once the kernel is loaded from USB, it no longer affects how the system runs at all.