One of the most common questions I’m asked is “Can I boot from SAN?”As with most technology questions, the answer is generally “It depends.”
Booting from SAN offers a great deal of flexibility and usually a significant performance bost when you’ve got the right hardware behind it.
My first experience with a Symmetrix was using an HP3000 on a Symmetrix 4.8. (We had an older Symm 3 at the time, but as a lowly operator (and a fairly uninformed one no less) I didn’t know it as much more than “the big cabinet”)
We had this old HP3000 running MPE/ix, which was HP’s old answer mainframe. I was working at Intuit and we ran 90% of our order processing through a single system at the time, and during tax time that was quite an amazing load for a system that had 8 100mhz PA-RISC processors. (remember, this was 1996)
So the HP3000 series 995/800 took two floor tiles by itself. Next to it were four racks of 2GB harddrives. The “spread it wide and not deep” mentality was alive and well at the time.
Enter the Symmetrix. We migrated all 80 or so 2GB harddrives to a single Symm frame, with, I believe it was 8GB of cache (which was a lot at the time) And most importantly, we were booting from the Symm.
We saw an amazing improvement. I mean something like 20 – 30 percent. It took me a while to realize (I was kind of daft at that point in time) that there were two reasons for this.
First, because the system was now paging to cache instead of to a physical volume write times that used to be measured in the 20ms to 30ms range were now being measured in the 5-10ms range. (Before you ask, this is an estimate based on my memory of marketing data provided to me over 10 years ago, so don’t quote me) The operating system can get a lot more done when it’s not waiting on virtual memory.
Second, because the disk mirroring that used to be handed by the operating system was now being handled by the symmetrix. As far as the OS was concerned it was connected to unprotected drives, and not having to expend any IO cycles doing multiple writes per transaction.
Third, reads and writes were distributed across many spindles, enabling the array to grab data from multiple sources at the same time.
Well the same holds true today, in fact even more so. While application performance demands have increased hundreds-fold, physical drives can only spin so fast, and as such can only provide data so fast irregardless of the speed of processors and memory.
So I’m in Kansas, working on setting up an IBM BladeCenter to boot from a Clariion CX3-20. There are better ways to go than Clariion when it comes to boot from SAN, HBA registration is required on the Clariion, and when you’re booting from SAN this has to be done manually before you install the OS. So it’s a lot of extra typing at first. (It’s also more typing when you have to replace an HBA, but that’s a different story.)The step-by-step, in case you’re interested is:
- Connect the HBA to the switch and enable the HBA BIOS.
- Boot the system (and let it time out, this forces the HBA to log into the swich)
- Zone the HBA to the required storage. (If your OS isn’t multi-path aware, you’ll need to zone *ONLY* to a single path at first, and enable the redundant paths after the fact.
- Assign the storage:
- (for Clariion) Reboot the host to force it to log into the Clariion.
- Register the HBA
- Assign it to a host
- assign the host to a Storage group with the boot lun in it.
- (for Symmetrix) Mask the HBA to the boot lun on a single path.
The downside to booting from SAN occurs especially in the windows environment. When a windows host loses access to the swap file, even for a second, you’re running the very real risk of BSOD’ing your system. This means that when the Clariion trespasses a device, it’s a crap shoot as to whether or not your system is going to stay up. The good news is that powerpath increases the timeout by adding it’s own timeout, so youre best of running powerpath, even in unlicensed mode, to handle the failover time..