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When Linux discovers new CCW devices, it causes a userspace application(hotplugd
or hald) to create the needed structures that represent
the devices. Linux creates structures for each discovered device,
regardless whether it will be used by the system or not. Because
usually devices are divided among different Linux systems and the
majority of devices in the whole environment is never used, this
process is often unnecessary for a great number of devices.
- Linux in an LPAR
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The IODCS contains the device definitions and possible
sharings for all LPARs. Because changing an IOCDS may
require an IML, the definitions often prefer sharing of
devices among LPARs even if not all of them are actively
using the full set. When Linux runs in the LPAR it attempts
to acquire information of all devices granted to the LPAR.
To prevent Linux from creating data structures for all
discovered devices defined in the IOCDS, use the cio_ignore
parameter to provide a blacklist for the Linux kernel.
This list contains addresses which can be ignored in the
above process.
This is no hard restriction because you can modify this
exclusion string at runtime via /proc/cio_ignore
if you need an additional device later on.
This support is available for all Linux 2.6 kernel based
distributions and newer.
- Linux as a z/VM guest
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- When Linux runs as a z/VM guest, Linux sees only the devices
explicitly granted to it by z/VM. Therefore, it won't need
the usage of the
cio_ignore interface.
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These are the benefits of using the cio_ignore
parameter:
- A well defined
cio_ignore parameter may prevent
problems related to out-of-memory-killer calls occurring
on boot time of low memory systems caused by all structures
created in the initial scan.
It saves memory that would otherwise be used by data structures
for every initialized device. Especially in low memory environments
this free RAM may significantly improve I/O performance
because of more space for the page cache (or the application
heap in other workloads).
- It speeds up the boot process because on boot the system
scans for all available devices.
These issues scale with the complexity of the environment (the
number of devices seen by the LPAR).
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