Date: Mon, 08 Oct 2012 17:14:20 +0200
From: Lucas B. Cohen l...@bnrlabs.com
Subject: what is an in-core disklabel ?
Hi,
I've seen the term in-core a couple times while reading up about BSD
disk labels. Does it refer to data that is cached in kernel memory ?
Context examples :
- fdisk(8) outputs parameters extracted from in-core disklabel
- bsdlabel(8)'s manual explains that the -n (dry run) parameter does
not install the new label either in-core or on-disk.
'cached' is not _technically_ exactly accurate, but you have the concept
basically correct.
The O/S reads the label information and stores it in an internal data
structure, Then, when it needs to use that data (frequently!:) it uses
the values in that internal structure, rather than attempting to re-read
from the disk, itself.
Technically, it's _not_ cached -- cached data is used to short-circuit
a 'read' attempt, using an in-memory block of byte instead of an actual
disk transfer.
The -effect- is similar, but there are *important* differences. 'Cache'
data is integrated with I/O operations, and a _write_ to the place where
the data was read from -invalidates- the cached data, whereupon, the next
read attempt will *not* be short-circuited, and the actual on-disk data
will be returned.
In the case of the disk label, it is read (once) into the internal data
structure, and only the internal data is used after that. A userland
app can change the 'on disk' data -- or trash it completely -- and what
the O/S thinks the label info is will NOT be affected by that change
to the 'on disk' data.
The warnings you see in the documentation, are reminders that the
O/S's 'internal' data and the 'on disk' data are *NOT* necessarily
the same. That looking at _one_ source of that data does *not* guarantee
that what you see =there= is the same as what is in the other place.
___
freebsd-questions@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-questions
To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
