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I need to boot multiple servers via PXE boot. They are all going to run the same ramdisk (i.e. "default" in the pxelinux.cfg directory), and then they all mount a root filesystem over NFS. Then they do a switch_root command to that NFS root image. I would like to make this NFS image read-only and use the same one for all of the servers, but then my servers are pretty much worthless as even /tmp is read-only.

I am thinking about leavings /tmp, /home and anything else that needs persistent unique storage off of the NFS image and instead mount those from local hard disks. Would this work? If so, how would I do it in the init script?

How is this done in the industry? I'm sure that there are racks and racks of servers out there that all need to be managed and updated from a single source. I can't be the only one who wants to do this, right? I want all of "user" data that is unique to a single server to be on the local hard disks, and all of the OS files (that need regular updates, etc.) to come from the PXE boot and NFS image.

Links to ample documentation on this type of server administration would be wonderful.

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What you want is some kind of union filesystem, where the NFS export serves as a read-only base, and a read-write overlay represents the delta. The delta can be stored in any other filesystem, whether on disk, USB stick, volatile memory, or a writable NFS export.

Linux supports several union filesystem implementations:

  • UnionFS (multiple implementations)
  • aufs (multiple versions)

It appears that aufs3 is preferred these days.

Linux live CDs employ a similar strategy; you can study how their init scripts work. An even better example to study is FAI, which PXE boots to an aufs root filesystem consisting of a read-only NFS share and a tmpfs image.

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