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For a school project I'm building a 10 node raspberry pi b cluster. I've done my research and know I need an sd card for every pi I use.
What I'm wondering though is when I get all the pi's connected can I link them all to store data to a single external harddrive? Almost like a reverse Raid system.
Yes you can. You can simply mount the hard drive to one of the Pi's as an extra drive, and then install Samba (sudo apt-get install samba) on every Pi. Configure the drive on the Pi it's mounted on as a shared Samba resource, and each Pi sees the drive - you can automount it in fstab if you want to.
For better performance (but possibly giving up the ability to mount the drive in other computers on your network such as Windows PCs) you can look into NFS to accomplish the same - this would allow full Linux/Unix ownership and permissions across the content.
Finally - you could also consider buying a NAS - many of them can be configured as an NFS mount - this way all Pis have the same configuration. The added benefit is that a NAS often allows RAID setup of multiple drives, so you get redundancy and fault-tolerance, as well as shared access to the drive. One NAS that is relatively inexpensive and does NFS is the Lenovo IX2.
Another lightweight solution would be to use weedfs -- it's just a tiny golang daemon and uses http to communicate between clients and servers -- you can either use curl or the weedfs executable to access your files.
