Monday, September 20, 2010

Introducing the Disk Puller

A cheap and effective way to hotplug disks automatically that I pioneered at Stratus while working on internal storage.

The basic idea is instead of letting the backplane or computer power  the drive, we do instead with our external power supply. According  to the storage controller's firmware, phy down == hotplug removed, which can be achieved by removing power to the target. This is 100% authentic and equal to walking up to a system and pulling/reinserting the disk by hand.

Parts list:
Great way to beat on MD, LVM, and the twisty maze that is the removal  path of the SCSI mid-layer.

It's pretty inexpensive, with a nice managed PDU and all the parts to automate 1 disk amounts to $80 per port. You can use the PDU for  other neat things too like say inserting and removing an external
USB hub, and it's connected peripherals, by controlling it's power source.

The equivalent functionality as part of an actual SAS/SATA target  emulator could cost you between 15-20K. The vendor whom I have the most experience with this product, who will remain nameless out of respect; required an exceptional amount of upkeep and would break for seemingly no reason at all. Instead of purchasing additional target emulators for fault injection, we instead installed disk pullers
everywhere for a fraction of the cost and with virtually no maintenance.