man/man4/geom.4

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.\" Copyright (c) 2002 Poul-Henning Kamp
.\" Copyright (c) 2002 Networks Associates Technology, Inc.
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.\" 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.
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.Dd March 27, 2002
.Os
.Dt GEOM 4
.Sh NAME
.Nm GEOM
.Nd modular disk I/O request transformation framework.
.Sh DESCRIPTION
The GEOM framework provides an infrastructure in which modules
can perform transformations on disk I/O requests on their path from
the upper kernel to the device drivers and back.
.Pp
Transformations in a GEOM context range from the simple geometric
displacement performed in typical disklabel modules over RAID
algorithms and device multipath resolution to full blown cryptographic
protection of the stored data.
.Pp
Compared to traditional "volume management", GEOM differs from most
and in some cases all previous implementations in the following ways:
.Bl -bullet
.It
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.
.It
GEOM is topologically agnostic.  Most volume management implementations
have very strict notions of how classes can fit together, very often
one fixed hierarchy is provided for instance  subdisk - plex -
volume.
.El
.Pp
Being extensible means that new transformations are treated no differently
than existing transformations.
.Pp
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.
.Pp
.Sh "TERMINOLOGY and TOPOLOGY"
Geom is quite object oriented and consequently the terminology
borrows a lot of context and semantics from the OO vocabulary:
.Pp
A "class", represented by the data structure g_class implements one
particular kind of transformation.  Typical examples are MBR disk
partition, BSD disklabel, and RAID5 classes.
.Pp
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.
.Pp
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.
.Pp
A "consumer" is the backdoor through which a geom connects to another
geom provider and through which I/O requests are sent.
.Pp
The topological relationship between these entities are as follows:
.Bl -bullet
.It
A class has zero or more geom instances.
.It
A geom has exactly one class it is derived from.
.It
A geom has zero or more consumers.
.It
A geom has zero or more providers.
.It
A consumer can be attached to zero or one providers.
.It
A provider can have zero or more consumers attached.
.El
.Pp
All geoms have a rank-number assigned, which is used to detect and
prevent loops in the acyclic directed graph.  This rank number is
assigned as follows:
.Bl -enum
.It
A geom with no attached consumers has rank=1
.It
A geom with attached consumers has a rank one higher than the
highest rank of the geoms of the providers its consumers are
attached to.
.El
.Sh "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
topological maneuvers exists to facilitate configuration and to
improve the overall flexibility.
.Pp
.Em TASTING
is a process that happens whenever a new class or new provider
is created and it is the class' 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.
.Pp
A new class will be offered to all existing providers in turn and a new
provider will be offered to all classes in turn.
.Pp
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:
.Bl -bullet
.It
Examine specific data structures on the disk.
.It
Examine properties like sectorsize or mediasize for the provider.
.It
Examine the rank number of the provider's geom.
.It
Examine the method name of the provider's geom.
.El
.Pp
.Em ORPHANIZATION
is the process by which a provider is removed while
it potentially is still being used.
.Pp
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
need to take appropriate action at that time.
.Pp
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.
.Pp
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.
.Pp
The typical scenario is
.Bl -bullet -offset indent -compact
.It
A device driver detects a disk has departed and orphans the provider for it.
.It
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
relevant pieces of the tree has heard the bad news.
.It
Eventually the buck stops when it reaches geom_dev at the top
of the stack.
.It
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.
.It
The geom whose provider is now attached will destroy the provider,
detach and destroy its consumer and destroy its geom.
.It
This process percolates all the way down through the mesh, until
the cleanup is complete.
.El
.Pp
While this approach seems byzantine, it does provide the maximum
flexibility and robustness in handling disappearing devices.
.Pp
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 and the geom event loop will stall.
.Pp
.Em 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.
.Pp
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.
.Pp
Now for the fine print:
.Pp
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 exclusive bit would render it impossible to open a
path through the MBR geom while "da0" is open for writing.
.Pp
From this it also follows that changing the size of open geoms can
only be done through their cooperation.
.Pp
Finally: the spoiling only happens when the write count goes from
zero to non-zero and the retasting only when the write count goes
back to zero.
.Pp
.Em 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.
.Pp
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 synchronization, 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.
.Pp
.Em 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.
.Pp
Finally IO is the reason we even do this: it concerns itself with
sending I/O requests through the graph.
.Pp
.Em "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 consumer.
Note that cloning the struct bio does not involve cloning the
actual data area specified in the IO request.
.Pp
In total five different IO requests exist in GEOM: read, write,
delete, format, get attribute, and set attribute.
.Pp
Read and write are self explanatory.
.Pp
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.
Technologies 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.
.Pp
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.
.Pp
Get attribute and set 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.
.Pp
(stay tuned while the author rests his brain and fingers: more to come.)
.Sh 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.
.Pp
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.
.Sh AUTHORS
.An "Poul-Henning Kamp" Aq phk@FreeBSD.org