Re: [PATCH/RFC] s390: Hypervisor File System

[Michael Holzheu <HOLZHEU@de.ibm.com>]

> > When the file system is mounted the accounting information is retrieved
> > and a file system tree is created with the attribute files containing
> > the cpu information. The content of the files remains unchanged until a
> > new update is  made. An update can be triggered from user space through
> > writing 'something' into a special purpose update file.
>
> Why are you caching the data?

We had two problems:

The first one was, that the hardware interface for getting the data is very
expensive. We always get back the data for all LPARs and all cpus.
Therefore
we do not want to get the data every time an attribute file is read.

The second problem was, that we want to provide a consistent snapshot
of the hypervisor data for the user space application. The mechanism to
achieve that is that the application first reads the update file. If
the data is too old, it triggers an update. Then we get the new
hypervisor data and update the whole filesystem tree. Then the
application can read file for file all data it needs. After all data
is read and the application wants to have a consistent snapshot,
it has to read the update attribute again. If the time value has changed,
another process has triggered another update in the meantime.
In this case the application has repeat the whole process.

Currently we allow only one update per second.

Michael

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