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GEOM - modular disk I/O request transformation framework



      GEOM - modular disk I/O request transformation framework


      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:
            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
            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 -
      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.
      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:
            A class has zero or more geom instances.
            A geom has exactly one class it is derived from.
            A geom has zero or more consumers.
            A geom has zero or more providers.
            A consumer can be attached to zero or one providers.
            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
      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 recognizes 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, will instantiate a geom to mul‐
      tiplex 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:
            Examine specific data structures on the disk.
            Examine properties like “sectorsize” or “mediasize” for the provider.
            Examine the rank number of the provider’s geom.
            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 event loop 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 self-destruct unless it has a way to keep functioning whilst lack‐
      ing the orphaned provider.  Geoms like disk slicers should therefore
      self-destruct whereas RAID5 or mirror geoms will be able to continue as
      long as they do not lose 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:
                  A device driver detects a disk has departed and orphans the
                provider for it.
                  The geoms on top of the disk receive the orphanization event
                and orphan 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 have heard the bad news.
                  Eventually the buck stops when it reaches geom_dev at the top
                of the stack.
                  Geom_dev will call destroy_dev(9) to stop any more requests
                from coming in.  It will sleep until any and all outstanding
                I/O requests have been returned.  It will explicitly close
                (i.e.: zero the access counts), a change which will propagate
                all the way down through the mesh.  It will then detach and
                destroy its geom.
                  The geom whose provider is now attached will destroy the
                provider, detach and destroy its consumer and destroy its geom.
                  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 of 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 an MBR geom provides da0s1 and
      da0s2, and on top of da0s1 a BSD geom provides da0s1a through da0s1e, and
      that both the MBR and BSD geoms have autoconfigured based on data struc‐
      tures 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 sys‐
      tem 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
      self-destruct as a result.  When da0 is closed, 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 thus rendering it impossible
      to open da0 for writing in that case.  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 happens only when the write count goes from
      non-zero to zero.
      INSERT/DELETE are very special operations which allow 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 being mounted as a
      file system.  Between the DEVFS geom’s 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 may 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 oper‐
      Finally, I/O is the reason we even do this: it concerns itself with send‐
      ing 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, are 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 I/O request.
      In total, four different I/O 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
      “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.)


      Several flags are provided for tracing GEOM operations and unlocking pro‐
      tection mechanisms via the kern.geom.debugflags sysctl.  All of these
      flags are off by default, and great care should be taken in turning them
      0x01 (G_T_TOPOLOGY)
              Provide tracing of topology change events.
      0x02 (G_T_BIO)
              Provide tracing of buffer I/O requests.
      0x04 (G_T_ACCESS)
              Provide tracing of access check controls.
      0x08 (unused)
      0x10 (allow foot shooting)
              Allow writing to Rank 1 providers.  This would, for example,
              allow the super-user to overwrite the MBR on the root disk or
              write random sectors elsewhere to a mounted disk.  The implica‐
              tions are obvious.
      0x40 (G_F_DISKIOCTL)
              This is unused at this time.
      0x80 (G_F_CTLDUMP)
              Dump contents of gctl requests.


      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.


      Poul-Henning Kamp 〈〉


Based on BSD UNIX
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