How memory is represented in sysfs

Both the core memory of a Linux instance and the available hotplug memory are represented by directories in sysfs.

The memory with which Linux is started is the core memory. On the running Linux system, additional memory can be added as hotplug memory. The Linux kernel requires core memory to allocate its own data structures.

In sysfs, both the core memory of a Linux instance and the available hotplug memory are represented in form of memory blocks of equal size. Each block is represented as a directory of the form /sys/devices/system/memory/memory<n>, where <n> is an integer. You can find out the block size by reading the /sys/devices/system/memory/block_size_bytes attribute.

In the naming scheme, the memory blocks with the lowest address ranges are assigned the lowest integer numbers. The core memory always begins with memory0. The hotplug memory blocks follow the core memory blocks.

You can calculate where the hotplug memory begins. To find the number of core memory blocks, divide the base memory by the block size.
  • With a core memory of 512 MB and a block size of 128 MB, the core memory is represented by four blocks, memory0 through memory3. Therefore, first hotplug memory block on this Linux instance is memory4.
  • Another Linux instance with a core memory of 1024 MB and access to the same hotplug memory, represents this first hotplug memory block as memory8.

The hotplug memory is available to all operating system instances within the z/VM® system or LPAR to which it was defined. The state sysfs attribute of a memory block indicates whether the block is in use by your own Linux system. The state attribute does not indicate whether a block is in use by another operating system instance. Attempts to add memory blocks that are already in use fail.