PHP & database sessions

If you have MySQL handle your PHP sessions and are finding that session data is not reliably being read after redirects via PHP’s header function, please read on.

For over a day I had been pulling my hair out trying to figure out why sessions were not sync’ing up after redirects via PHP’s header function. Some pages would load and others would not. I had just finished migrating our code base to a new server running slightly different versions of Apache, MySQL, and PHP – though from the same branches.

After some expirimentation, I discovered sessions handled by the defalt file handler would work just fine – just not reliadbly through my custom database handler. After even further investigation, redirects were working, the data for the session was “late” showing up and by simply refreshing the screen the data would be magically populated in $_SESSION.

After some reading around on the internet and some deep thinking, I understood the problem to be an issue of timing and/or cache timing issues. So, to resolve the issue, I add the session_write_close() command before each redirect I need. It works. Although, do not ask me why.

My LAMP versions:
Linux: RHEL 4.0
Apache: 2.0.52-22.ent
MySQL: 4.1.12-3.RHEL4.1
PHP: 4.3.9-3.9

Hope this saves someone!

Linux + Onboard SATA RAID

Are you considering using the onboard RAID feature of your SATA controller? Before you dive head-on into this project – there are a few things you should know.

Recently, my job took me into territory that was somewhat unfamiliar – the server realm. When I say server, I don’t mean a home server most of us Linux enthusiasts have. I mean a server – dual Opteron, 8GB RAM, and 2 x 320GB hard drives. The kind of server that can handle complex database queries in a matter of a few nano seconds. The kind of server that must be 100% secure and reliable. Our hope was to run the drives in a RAID-1 configuration for maximum stability. For those unfamiliar with RAID, a RAID-1 array mirrors data from one drive to another. This can really speed up disk reads and also gives you a very stable environment when using failover support – i.e. if one drive happens to die on you, the OS will automatically rollover to the backup disk. I had never setup a RAID array before, but felt confident that it could not be too difficult – of course, I was wrong.

The motherboard we selected for our server (a Tyan S2881) has four SATA connectors and a RAID controller. On initial power-up, I entered the BIOS and enabled the RAID controller for the SATA. Once RAID had been enabled, I entered the RAID BIOS and created the RAID-1 array using both disks. Assuming the RAID controller would handle everything else and that Linux would detect it, I continued with my installation of RedHat Enterprise Linux 4.0

I was gravely mistaken. Here is the issue: most newer motherboards that claim to have RAID support, have what is known as FakeRAID – essentially a software RAID. Drivers for these FakeRAID controllers found on most motherboards is not really supported in newer kernels. In my case, a Silicon Image 3114 controller, had a driver in the kernel but only supported the standard SATA protocol and did not enable any of the FakeRAID features of this chipset. Reading on boards and forums around the ‘net, I was informed that disabling the RAID controller in the BIOS and simply using the built-in software RAID within Linux was a much better alternative and was no better or worse than using the FakeRAID for my chipset.

Unsure if these claims were true, I spent some time finding a linux driver for my Silicon Image 3114 RAID controller. The manufacturer had them posted on their website along with installation directions. Half a work day was spent wrestling with these drivers reformating, repartionting. In the end I abandoned the idea of using the onboard RAID controller and proceeded to use the Software RAID option found in RedHat Linux.

After a successful install of RedHat Enterprise Linux 4 onto a RAID-1 array, I found that the boot loader was not correctly installed. If you run into the case where, after a successful install of Linux onto a RAID array, you are presented with simply: “GRUB” at boot up – do not worry. Grab your install CD or DVD and boot using “linux rescue”. Once booted, run:

grub
root (hd0,0)
setup (hd0)

Now reboot and you should be in business. Don’t forget to remove the install CD or DVD from the drive.

Heed my words, unless the RAID controller on your motherboard is a true hardware RAID controller, do not waste your time trying to get the FakeRAID working.