Date: Mon, 8 Oct 2012 11:22:52 -0500 (CDT) From: Robert Bonomi <bonomi@mail.r-bonomi.com> To: freebsd-questions@freebsd.org, lbc@bnrlabs.com Subject: Re: what is an "in-core" disklabel ? Message-ID: <201210081622.q98GMqLH029828@mail.r-bonomi.com> In-Reply-To: <5072EDCC.4040303@bnrlabs.com>
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> Date: Mon, 08 Oct 2012 17:14:20 +0200 > From: "Lucas B. Cohen" <lbc@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.
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