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GEOM(4)		       FreeBSD Kernel Interfaces Manual		       GEOM(4)

NAME
     GEOM -- modular disk I/O request transformation framework.

DESCRIPTION
     The GEOM framework	provides an infrastructure in which "classes" can per-
     form transformations on disk I/O requests on their	path from the upper
     kernel to the device drivers and back.

     Transformations in	a GEOM context range from the simple geometric dis-
     placement performed in typical disk partitioning modules over RAID	algo-
     rithms and	device multipath resolution to full blown cryptographic	pro-
     tection of	the stored data.

     Compared to traditional "volume management", GEOM differs from most and
     in	some cases all previous	implementations	in the following ways:

     +o	 GEOM is extensible.  It is trivially simple to	write a	new class of
	 transformation	and it will not	be given stepchild treatment.  If
	 someone for some reason wanted	to mount IBM MVS diskpacks, a class
	 recognizing and configuring their VTOC	information would be a trivial
	 matter.

     +o	 GEOM is topologically agnostic.  Most volume management implementa-
	 tions have very strict	notions	of how classes can fit together, very
	 often one fixed hierarchy is provided for instance  subdisk - plex -
	 volume.

     Being extensible means that new transformations are treated no differ-
     ently than	existing transformations.

     Fixed hierarchies are bad because they make it impossible to express the
     intent efficiently.  In the fixed hierarchy above it is not possible to
     mirror two	physical disks and then	partition the mirror into subdisks,
     instead one is forced to make subdisks on the physical volumes and	to
     mirror these two and two resulting	in a much more complex configuration.
     GEOM on the other hand does not care in which order things	are done, the
     only restriction is that cycles in	the graph will not be allowed.

TERMINOLOGY and	TOPOLOGY
     Geom is quite object oriented and consequently the	terminology borrows a
     lot of context and	semantics from the OO vocabulary:

     A "class",	represented by the data	structure g_class implements one par-
     ticular kind of transformation.  Typical examples are MBR disk partition,
     BSD disklabel, and	RAID5 classes.

     An	instance of a class is called a	"geom" and represented by the data
     structure "g_geom".  In a typical i386 FreeBSD system, there will be one
     geom of class MBR for each	disk.

     A "provider", represented by the data structure "g_provider", is the
     front gate	at which a geom	offers service.	 A provider is "a disk-like
     thing which appears in /dev" - a logical disk in other words.  All
     providers have three main properties: name, sectorsize and	size.

     A "consumer" is the backdoor through which	a geom connects	to another
     geom provider and through which I/O requests are sent.

     The topological relationship between these	entities are as	follows:

     +o	 A class has zero or more geom instances.

     +o	 A geom	has exactly one	class it is derived from.

     +o	 A geom	has zero or more consumers.

     +o	 A geom	has zero or more providers.

     +o	 A consumer can	be attached to zero or one providers.

     +o	 A provider can	have zero or more consumers attached.

     All geoms have a rank-number assigned, which is used to detect and	pre-
     vent loops	in the acyclic directed	graph.	This rank number is assigned
     as	follows:

     1.	  A geom with no attached consumers has	rank=1

     2.	  A geom with attached consumers has a rank one	higher than the	high-
	  est rank of the geoms	of the providers its consumers are attached
	  to.

SPECIAL	TOPOLOGICAL MANEUVERS
     In	addition to the	straightforward	attach,	which attaches a consumer to a
     provider, and detach, which breaks	the bond, a number of special topolog-
     ical maneuvers exists to facilitate configuration and to improve the
     overall flexibility.

     TASTING is	a process that happens whenever	a new class or new provider is
     created and it provides the class a chance	to automatically configure an
     instance on providers, which it recognize as its own.  A typical example
     is	the MBR	disk-partition class which will	look for the MBR table in the
     first sector and if found and validated it	will instantiate a geom	to
     multiplex according to the	contents of the	MBR.

     A new class will be offered to all	existing providers in turn and a new
     provider will be offered to all classes in	turn.

     Exactly what a class does to recognize if it should accept	the offered
     provider is not defined by	GEOM, but the sensible set of options are:

     +o	 Examine specific data structures on the disk.

     +o	 Examine properties like sectorsize or mediasize for the provider.

     +o	 Examine the rank number of the	provider's geom.

     +o	 Examine the method name of the	provider's geom.

     ORPHANIZATION is the process by which a provider is removed while it
     potentially is still being	used.

     When a geom orphans a provider, all future	I/O requests will "bounce" on
     the provider with an error	code set by the	geom.  Any consumers attached
     to	the provider will receive notification about the orphanization when
     the eventloop gets	around to it, and they can take	appropriate action at
     that time.

     A geom which came into being as a result of a normal taste	operation
     should selfdestruct unless	it has a way to	keep functioning lacking the
     orphaned provider.	 Geoms like diskslicers	should therefore selfdestruct
     whereas RAID5 or mirror geoms will	be able	to continue, as	long as	they
     do	not loose quorum.

     When a provider is	orphaned, this does not	necessarily result in any
     immediate change in the topology: any attached consumers are still
     attached, any opened paths	are still open,	any outstanding	I/O requests
     are still outstanding.

     The typical scenario is
	   +o   A device	driver detects a disk has departed and orphans the
	       provider	for it.
	   +o   The geoms on top	of the disk receive the	orphanization event
	       and orphans all their providers in turn.	 Providers, which are
	       not attached to,	will typically self-destruct right away.  This
	       process continues in a quasi-recursive fashion until all	rele-
	       vant pieces of the tree has heard the bad news.
	   +o   Eventually the buck stops when it reaches geom_dev at the top
	       of the stack.
	   +o   Geom_dev	will call destroy_dev(9) to stop any more request from
	       coming in.  It will sleep until all (if any) outstanding	I/O
	       requests	have been returned.  It	will explicitly	close (ie:
	       zero the	access counts),	a change which will propagate all the
	       way down	through	the mesh.  It will then	detach and destroy its
	       geom.
	   +o   The geom	whose provider is now attached will destroy the
	       provider, detach	and destroy its	consumer and destroy its geom.
	   +o   This process percolates all the way down	through	the mesh,
	       until the cleanup is complete.

     While this	approach seems byzantine, it does provide the maximum flexi-
     bility and	robustness in handling disappearing devices.

     The one absolutely	crucial	detail to be aware is that if the device
     driver does not return all	I/O requests, the tree will not	unravel.

     SPOILING is a special case	of orphanization used to protect against stale
     metadata.	It is probably easiest to understand spoiling by going through
     an	example.

     Imagine a disk, "da0" on top of which a MBR geom provides "da0s1" and
     "da0s2" and on top	of "da0s1" a BSD geom provides "da0s1a"	through
     "da0s1e", both the	MBR and	BSD geoms have autoconfigured based on data
     structures	on the disk media.  Now	imagine	the case where "da0" is	opened
     for writing and those data	structures are modified	or overwritten:	 Now
     the geoms would be	operating on stale metadata unless some	notification
     system can	inform them otherwise.

     To	avoid this situation, when the open of "da0" for write happens,	all
     attached consumers	are told about this, and geoms like MBR	and BSD	will
     selfdestruct as a result.	When "da0" is closed again, it will be offered
     for tasting again and if the data structures for MBR and BSD are still
     there, new	geoms will instantiate themselves anew.

     Now for the fine print:

     If	any of the paths through the MBR or BSD	module were open, they would
     have opened downwards with	an exclusive bit rendering it impossible to
     open "da0"	for writing in that case and conversely	the requested exclu-
     sive bit would render it impossible to open a path	through	the MBR	geom
     while "da0" is open for writing.

     From this it also follows that changing the size of open geoms can	only
     be	done with their	cooperation.

     Finally: the spoiling only	happens	when the write count goes from zero to
     non-zero and the retasting	only when the write count goes from non-zero
     to	zero.

     INSERT/DELETE are a very special operation	which allows a new geom	to be
     instantiated between a consumer and a provider attached to	each other and
     to	remove it again.

     To	understand the utility of this,	imagine	a provider with	being mounted
     as	a file system.	Between	the DEVFS geoms	consumer and its provider we
     insert a mirror module which configures itself with one mirror copy and
     consequently is transparent to the	I/O requests on	the path.  We can now
     configure yet a mirror copy on the	mirror geom, request a synchroniza-
     tion, and finally drop the	first mirror copy.  We have now	in essence
     moved a mounted file system from one disk to another while	it was being
     used.  At this point the mirror geom can be deleted from the path again,
     it	has served its purpose.

     CONFIGURE is the process where the	administrator issues instructions for
     a particular class	to instantiate itself.	There are multiple ways	to
     express intent in this case, a particular provider	can be specified with
     a level of	override forcing for instance a	BSD disklabel module to	attach
     to	a provider which was not found palatable during	the TASTE operation.

     Finally IO	is the reason we even do this: it concerns itself with sending
     I/O requests through the graph.

     I/O REQUESTS represented by struct	bio, originate at a consumer, are
     scheduled on its attached provider, and when processed, returned to the
     consumer.	It is important	to realize that	the struct bio which enters
     through the provider of a particular geom does not	"come out on the other
     side".  Even simple transformations like MBR and BSD will clone the
     struct bio, modify	the clone, and schedule	the clone on their own con-
     sumer.  Note that cloning the struct bio does not involve cloning the
     actual data area specified	in the IO request.

     In	total four different IO	requests exist in GEOM:	read, write, delete,
     and get attribute.

     Read and write are	self explanatory.

     Delete indicates that a certain range of data is no longer	used and that
     it	can be erased or freed as the underlying technology supports.  Tech-
     nologies like flash adaptation layers can arrange to erase	the relevant
     blocks before they	will become reassigned and cryptographic devices may
     want to fill random bits into the range to	reduce the amount of data
     available for attack.

     It	is important to	recognize that a delete	indication is not a request
     and consequently there is no guarantee that the data actually will	be
     erased or made unavailable	unless guaranteed by specific geoms in the
     graph.  If	"secure	delete"	semantics are required,	a geom should be
     pushed which converts delete indications into (a sequence of) write
     requests.

     Get attribute supports inspection and manipulation	of out-of-band
     attributes	on a particular	provider or path.  Attributes are named	by
     ascii strings and they will be discussed in a separate section below.

     (stay tuned while the author rests	his brain and fingers: more to come.)

HISTORY
     This software was developed for the FreeBSD Project by Poul-Henning Kamp
     and NAI Labs, the Security	Research Division of Network Associates, Inc.
     under DARPA/SPAWAR	contract N66001-01-C-8035 ("CBOSS"), as	part of	the
     DARPA CHATS research program.

     The first precursor for GEOM was a	gruesome hack to Minix 1.2 and was
     never distributed.	 An earlier attempt to implement a less	general	scheme
     in	FreeBSD	never succeeded.

AUTHORS
     Poul-Henning Kamp <phk@FreeBSD.org>

FreeBSD	9.2			March 27, 2002			   FreeBSD 9.2

NAME | DESCRIPTION | TERMINOLOGY and TOPOLOGY | SPECIAL TOPOLOGICAL MANEUVERS | HISTORY | AUTHORS

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