path: root/documentation/content/en/articles/geom-class/_index.po

# SOME DESCRIPTIVE TITLE
# Copyright (C) YEAR The FreeBSD Project
# This file is distributed under the same license as the FreeBSD Documentation package.
# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.
#
#, fuzzy
msgid ""
msgstr ""
"Project-Id-Version: FreeBSD Documentation VERSION\n"
"POT-Creation-Date: 2022-02-01 09:21-0300\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
"Language-Team: LANGUAGE <LL@li.org>\n"
"Language: \n"
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"

#. type: YAML Front Matter: description
#: documentation/content/en/articles/geom-class/_index.adoc:1
#, no-wrap
msgid "A guide to GEOM internals, and writing your own GEOM class"
msgstr ""

#. type: Title =
#: documentation/content/en/articles/geom-class/_index.adoc:1
#: documentation/content/en/articles/geom-class/_index.adoc:11
#, no-wrap
msgid "Writing a GEOM Class"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:44
msgid "Abstract"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:47
msgid ""
"This text documents some starting points in developing GEOM classes, and "
"kernel modules in general.  It is assumed that the reader is familiar with C "
"userland programming."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:49
msgid "'''"
msgstr ""

#. type: Title ==
#: documentation/content/en/articles/geom-class/_index.adoc:53
#, no-wrap
msgid "Introduction"
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:56
#, no-wrap
msgid "Documentation"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:60
msgid ""
"Documentation on kernel programming is scarce - it is one of few areas where "
"there is nearly nothing in the way of friendly tutorials, and the phrase "
"\"use the source!\" really holds true.  However, there are some bits and "
"pieces (some of them seriously outdated) floating around that should be "
"studied before beginning to code:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:62
msgid ""
"The extref:{developers-handbook}[FreeBSD Developer's Handbook] - part of the "
"documentation project, it does not contain anything specific to kernel "
"programming, but rather some general useful information."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:63
msgid ""
"The extref:{arch-handbook}[FreeBSD Architecture Handbook] - also from the "
"documentation project, contains descriptions of several low-level facilities "
"and procedures. The most important chapter is 13, extref:{arch-handbook}"
"[Writing FreeBSD device drivers, driverbasics]."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:64
msgid ""
"The Blueprints section of http://www.freebsddiary.org[FreeBSD Diary] web "
"site - contains several interesting articles on kernel facilities."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:65
msgid ""
"The man pages in section 9 - for important documentation on kernel functions."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:66
msgid ""
"The man:geom[4] man page and http://phk.freebsd.dk/pubs/[PHK's GEOM slides] "
"- for general introduction of the GEOM subsystem."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:67
msgid ""
"Man pages man:g_bio[9], man:g_event[9], man:g_data[9], man:g_geom[9], man:"
"g_provider[9], man:g_consumer[9], man:g_access[9] & others linked from "
"those, for documentation on specific functionalities."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:68
msgid ""
"The man:style[9] man page - for documentation on the coding-style "
"conventions which must be followed for any code which is to be committed to "
"the FreeBSD tree."
msgstr ""

#. type: Title ==
#: documentation/content/en/articles/geom-class/_index.adoc:70
#, no-wrap
msgid "Preliminaries"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:77
msgid ""
"The best way to do kernel development is to have (at least) two separate "
"computers.  One of these would contain the development environment and "
"sources, and the other would be used to test the newly written code by "
"network-booting and network-mounting filesystems from the first one.  This "
"way if the new code contains bugs and crashes the machine, it will not mess "
"up the sources (and other \"live\" data).  The second system does not even "
"require a proper display.  Instead, it could be connected with a serial "
"cable or KVM to the first one."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:80
msgid ""
"But, since not everybody has two or more computers handy, there are a few "
"things that can be done to prepare an otherwise \"live\" system for "
"developing kernel code.  This setup is also applicable for developing in a "
"http://www.vmware.com/[VMWare] or http://www.qemu.org/[QEmu] virtual machine "
"(the next best thing after a dedicated development machine)."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:82
#, no-wrap
msgid "Modifying a System for Development"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:85
msgid ""
"For any kernel programming a kernel with `INVARIANTS` enabled is a must-"
"have. So enter these in your kernel configuration file:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:90
#, no-wrap
msgid ""
"options INVARIANT_SUPPORT\n"
"options INVARIANTS\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:93
msgid ""
"For more debugging you should also include WITNESS support, which will alert "
"you of mistakes in locking:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:98
#, no-wrap
msgid ""
"options WITNESS_SUPPORT\n"
"options WITNESS\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:101
msgid "For debugging crash dumps, a kernel with debug symbols is needed:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:105
#, no-wrap
msgid "  makeoptions    DEBUG=-g\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:110
msgid ""
"With the usual way of installing the kernel (`make installkernel`) the debug "
"kernel will not be automatically installed.  It is called [.filename]#kernel."
"debug# and located in [.filename]#/usr/obj/usr/src/sys/KERNELNAME/#.  For "
"convenience it should be copied to [.filename]#/boot/kernel/#."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:113
msgid ""
"Another convenience is enabling the kernel debugger so you can examine a "
"kernel panic when it happens.  For this, enter the following lines in your "
"kernel configuration file:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:119
#, no-wrap
msgid ""
"options KDB\n"
"options DDB\n"
"options KDB_TRACE\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:122
msgid ""
"For this to work you might need to set a sysctl (if it is not on by default):"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:126
#, no-wrap
msgid "  debug.debugger_on_panic=1\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:134
msgid ""
"Kernel panics will happen, so care should be taken with the filesystem "
"cache.  In particular, having softupdates might mean the latest file version "
"could be lost if a panic occurs before it is committed to storage.  "
"Disabling softupdates yields a great performance hit, and still does not "
"guarantee data consistency.  Mounting filesystem with the \"sync\" option is "
"needed for that.  For a compromise, the softupdates cache delays can be "
"shortened.  There are three sysctl's that are useful for this (best to be "
"set in [.filename]#/etc/sysctl.conf#):"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:140
#, no-wrap
msgid ""
"kern.filedelay=5\n"
"kern.dirdelay=4\n"
"kern.metadelay=3\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:143
msgid "The numbers represent seconds."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:151
msgid ""
"For debugging kernel panics, kernel core dumps are required.  Since a kernel "
"panic might make filesystems unusable, this crash dump is first written to a "
"raw partition.  Usually, this is the swap partition.  This partition must be "
"at least as large as the physical RAM in the machine.  On the next boot, the "
"dump is copied to a regular file.  This happens after filesystems are "
"checked and mounted, and before swap is enabled.  This is controlled with "
"two [.filename]#/etc/rc.conf# variables:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:156
#, no-wrap
msgid ""
"dumpdev=\"/dev/ad0s4b\"\n"
"dumpdir=\"/usr/core\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:159
msgid ""
"The `dumpdev` variable specifies the swap partition and `dumpdir` tells the "
"system where in the filesystem to relocate the core dump on reboot."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:162
msgid ""
"Writing kernel core dumps is slow and takes a long time so if you have lots "
"of memory (>256M) and lots of panics it could be frustrating to sit and wait "
"while it is done (twice - first to write it to swap, then to relocate it to "
"filesystem).  It is convenient then to limit the amount of RAM the system "
"will use via a [.filename]#/boot/loader.conf# tunable:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:166
#, no-wrap
msgid "  hw.physmem=\"256M\"\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:169
msgid ""
"If the panics are frequent and filesystems large (or you simply do not trust "
"softupdates+background fsck) it is advisable to turn background fsck off via "
"[.filename]#/etc/rc.conf# variable:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:173
#, no-wrap
msgid "  background_fsck=\"NO\"\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:178
msgid ""
"This way, the filesystems will always get checked when needed.  Note that "
"with background fsck, a new panic could happen while it is checking the "
"disks.  Again, the safest way is not to have many local filesystems by using "
"another computer as an NFS server."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:180
#, no-wrap
msgid "Starting the Project"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:184
msgid ""
"For the purpose of creating a new GEOM class, an empty subdirectory has to "
"be created under an arbitrary user-accessible directory.  You do not have to "
"create the module directory under [.filename]#/usr/src#."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:186
#, no-wrap
msgid "The Makefile"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:189
msgid ""
"It is good practice to create [.filename]#Makefiles# for every nontrivial "
"coding project, which of course includes kernel modules."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:192
msgid ""
"Creating the [.filename]#Makefile# is simple thanks to an extensive set of "
"helper routines provided by the system.  In short, here is how a minimal [."
"filename]#Makefile# looks for a kernel module:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:197
#, no-wrap
msgid ""
"SRCS=g_journal.c\n"
"KMOD=geom_journal\n"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:199
#, no-wrap
msgid ".include <bsd.kmod.mk>\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:203
msgid ""
"This [.filename]#Makefile# (with changed filenames) will do for any kernel "
"module, and a GEOM class can reside in just one kernel module.  If more than "
"one file is required, list it in the `SRCS` variable, separated with "
"whitespace from other filenames."
msgstr ""

#. type: Title ==
#: documentation/content/en/articles/geom-class/_index.adoc:205
#, no-wrap
msgid "On FreeBSD Kernel Programming"
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:208
#, no-wrap
msgid "Memory Allocation"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:213
msgid ""
"See man:malloc[9].  Basic memory allocation is only slightly different than "
"its userland equivalent.  Most notably, `malloc`() and `free`() accept "
"additional parameters as is described in the man page."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:215
msgid ""
"A \"malloc type\" must be declared in the declaration section of a source "
"file, like this:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:219
#, no-wrap
msgid "  static MALLOC_DEFINE(M_GJOURNAL, \"gjournal data\", \"GEOM_JOURNAL Data\");\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:222
msgid ""
"To use this macro, [.filename]#sys/param.h#, [.filename]#sys/kernel.h# and [."
"filename]#sys/malloc.h# headers must be included."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:225
msgid ""
"There is another mechanism for allocating memory, the UMA (Universal Memory "
"Allocator).  See man:uma[9] for details, but it is a special type of "
"allocator mainly used for speedy allocation of lists comprised of same-sized "
"items (for example, dynamic arrays of structs)."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:227
#, no-wrap
msgid "Lists and Queues"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:234
msgid ""
"See man:queue[3].  There are a LOT of cases when a list of things needs to "
"be maintained.  Fortunately, this data structure is implemented (in several "
"ways) by C macros included in the system.  The most used list type is TAILQ "
"because it is the most flexible.  It is also the one with largest memory "
"requirements (its elements are doubly-linked) and also the slowest (although "
"the speed variation is on the order of several CPU instructions more, so it "
"should not be taken seriously)."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:236
msgid ""
"If data retrieval speed is very important, see man:tree[3] and man:"
"hashinit[9]."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:238
#, no-wrap
msgid "BIOs"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:242
msgid ""
"Structure `bio` is used for any and all Input/Output operations concerning "
"GEOM.  It basically contains information about what device ('provider') "
"should satisfy the request, request type, offset, length, pointer to a "
"buffer, and a bunch of \"user-specific\" flags and fields that can help "
"implement various hacks."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:246
msgid ""
"The important thing here is that ``bio``s are handled asynchronously.  That "
"means that, in most parts of the code, there is no analogue to userland's "
"man:read[2] and man:write[2] calls that do not return until a request is "
"done.  Rather, a developer-supplied function is called as a notification "
"when the request gets completed (or results in error)."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:250
msgid ""
"The asynchronous programming model (also called \"event-driven\") is "
"somewhat harder than the much more used imperative one used in userland (at "
"least it takes a while to get used to it).  In some cases the helper "
"routines `g_write_data`() and `g_read_data`() can be used, but __not "
"always__.  In particular, they cannot be used when a mutex is held; for "
"example, the GEOM topology mutex or the internal mutex held during the `."
"start`() and `.stop`() functions."
msgstr ""

#. type: Title ==
#: documentation/content/en/articles/geom-class/_index.adoc:252
#, no-wrap
msgid "On GEOM Programming"
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:255
#, no-wrap
msgid "Ggate"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:259
msgid ""
"If maximum performance is not needed, a much simpler way of making a data "
"transformation is to implement it in userland via the ggate (GEOM gate) "
"facility.  Unfortunately, there is no easy way to convert between, or even "
"share code between the two approaches."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:261
#, no-wrap
msgid "GEOM Class"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:266
msgid ""
"GEOM classes are transformations on the data.  These transformations can be "
"combined in a tree-like fashion.  Instances of GEOM classes are called "
"__geoms__."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:268
msgid ""
"Each GEOM class has several \"class methods\" that get called when there is "
"no geom instance available (or they are simply not bound to a single "
"instance):"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:270
msgid ""
"`.init` is called when GEOM becomes aware of a GEOM class (when the kernel "
"module gets loaded.)"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:271
msgid ""
"`.fini` gets called when GEOM abandons the class (when the module gets "
"unloaded)"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:272
msgid ""
"`.taste` is called next, once for each provider the system has available. If "
"applicable, this function will usually create and start a geom instance."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:273
msgid "`.destroy_geom` is called when the geom should be disbanded"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:274
msgid ""
"`.ctlconf` is called when user requests reconfiguration of existing geom"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:276
msgid ""
"Also defined are the GEOM event functions, which will get copied to the geom "
"instance."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:278
msgid ""
"Field `.geom` in the `g_class` structure is a LIST of geoms instantiated "
"from the class."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:280
msgid "These functions are called from the g_event kernel thread."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:282
#, no-wrap
msgid "Softc"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:288
msgid ""
"The name \"softc\" is a legacy term for \"driver private data\".  The name "
"most probably comes from the archaic term \"software control block\".  In "
"GEOM, it is a structure (more precise: pointer to a structure) that can be "
"attached to a geom instance to hold whatever data is private to the geom "
"instance.  Most GEOM classes have the following members:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:290
msgid "`struct g_provider *provider` : The \"provider\" this geom instantiates"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:291
msgid "`uint16_t n_disks` : Number of consumer this geom consumes"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:292
msgid ""
"`struct g_consumer \\**disks` : Array of `struct g_consumer*`. (It is not "
"possible to use just single indirection because struct g_consumer* are "
"created on our behalf by GEOM)."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:295
msgid ""
"The `softc` structure contains all the state of geom instance.  Every geom "
"instance has its own softc."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:297
#, no-wrap
msgid "Metadata"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:300
msgid ""
"Format of metadata is more-or-less class-dependent, but MUST start with:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:302
msgid "16 byte buffer for null-terminated signature (usually the class name)"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:303
msgid "uint32 version ID"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:305
msgid ""
"It is assumed that geom classes know how to handle metadata with version "
"ID's lower than theirs."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:307
msgid ""
"Metadata is located in the last sector of the provider (and thus must fit in "
"it)."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:309
msgid ""
"(All this is implementation-dependent but all existing code works like that, "
"and it is supported by libraries.)"
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:311
#, no-wrap
msgid "Labeling/creating a GEOM"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:314
msgid "The sequence of events is:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:316
msgid "user calls man:geom[8] utility (or one of its hardlinked friends)"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:317
msgid ""
"the utility figures out which geom class it is supposed to handle and "
"searches for [.filename]#geom_CLASSNAME.so# library (usually in [.filename]#/"
"lib/geom#)."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:318
msgid ""
"it man:dlopen[3]-s the library, extracts the definitions of command-line "
"parameters and helper functions."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:320
msgid "In the case of creating/labeling a new geom, this is what happens:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:322
msgid ""
"man:geom[8] looks in the command-line argument for the command (usually "
"`label`), and calls a helper function."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:323
msgid ""
"The helper function checks parameters and gathers metadata, which it "
"proceeds to write to all concerned providers."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:324
msgid ""
"This \"spoils\" existing geoms (if any) and initializes a new round of "
"\"tasting\" of the providers. The intended geom class recognizes the "
"metadata and brings the geom up."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:326
msgid ""
"(The above sequence of events is implementation-dependent but all existing "
"code works like that, and it is supported by libraries.)"
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:328
#, no-wrap
msgid "GEOM Command Structure"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:332
msgid ""
"The helper [.filename]#geom_CLASSNAME.so# library exports `class_commands` "
"structure, which is an array of `struct g_command` elements.  Commands are "
"of uniform format and look like:"
msgstr ""

#. type: delimited block . 4
#: documentation/content/en/articles/geom-class/_index.adoc:336
#, no-wrap
msgid "  verb [-options] geomname [other]\n"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:339
msgid "Common verbs are:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:341
msgid ""
"label - to write metadata to devices so they can be recognized at tasting "
"and brought up in geoms"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:342
msgid "destroy - to destroy metadata, so the geoms get destroyed"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:344
msgid "Common options are:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:346
msgid "`-v` : be verbose"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:347
msgid "`-f` : force"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:351
msgid ""
"Many actions, such as labeling and destroying metadata can be performed in "
"userland.  For this, `struct g_command` provides field `gc_func` that can be "
"set to a function (in the same [.filename]#.so#) that will be called to "
"process a verb.  If `gc_func` is NULL, the command will be passed to kernel "
"module, to `.ctlreq` function of the geom class."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:353
#, no-wrap
msgid "Geoms"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:357
msgid ""
"Geoms are instances of GEOM classes.  They have internal data (a softc "
"structure) and some functions with which they respond to external events."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:359
msgid "The event functions are:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:361
msgid "`.access` : calculates permissions (read/write/exclusive)"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:362
msgid "`.dumpconf` : returns XML-formatted information about the geom"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:363
msgid "`.orphan` : called when some underlying provider gets disconnected"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:364
msgid "`.spoiled` : called when some underlying provider gets written to"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:365
msgid "`.start` : handles I/O"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:367
msgid ""
"These functions are called from the `g_down` kernel thread and there can be "
"no sleeping in this context, (see definition of sleeping elsewhere) which "
"limits what can be done quite a bit, but forces the handling to be fast."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:369
msgid ""
"Of these, the most important function for doing actual useful work is the `."
"start`() function, which is called when a BIO request arrives for a provider "
"managed by a instance of geom class."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:371
#, no-wrap
msgid "GEOM Threads"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:374
msgid "There are three kernel threads created and run by the GEOM framework:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:376
msgid ""
"`g_down` : Handles requests coming from high-level entities (such as a "
"userland request) on the way to physical devices"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:377
msgid ""
"`g_up` : Handles responses from device drivers to requests made by higher-"
"level entities"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:378
msgid ""
"`g_event` : Handles all other cases: creation of geom instances, access "
"counting, \"spoil\" events, etc."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:380
msgid ""
"When a user process issues \"read data X at offset Y of a file\" request, "
"this is what happens:"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:382
msgid ""
"The filesystem converts the request into a struct bio instance and passes it "
"to the GEOM subsystem. It knows what geom instance should handle it because "
"filesystems are hosted directly on a geom instance."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:383
msgid ""
"The request ends up as a call to the `.start`() function made on the g_down "
"thread and reaches the top-level geom instance."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:384
msgid ""
"This top-level geom instance (for example the partition slicer) determines "
"that the request should be routed to a lower-level instance (for example the "
"disk driver). It makes a copy of the bio request (bio requests _ALWAYS_ need "
"to be copied between instances, with `g_clone_bio`()!), modifies the data "
"offset and target provider fields and executes the copy with `g_io_request`()"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:385
msgid ""
"The disk driver gets the bio request also as a call to `.start`() on the "
"`g_down` thread. It talks to hardware, gets the data back, and calls "
"`g_io_deliver`() on the bio."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:386
msgid ""
"Now, the notification of bio completion \"bubbles up\" in the `g_up` thread. "
"First the partition slicer gets `.done`() called in the `g_up` thread, it "
"uses information stored in the bio to free the cloned `bio` structure (with "
"`g_destroy_bio`()) and calls `g_io_deliver`() on the original request."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:387
msgid "The filesystem gets the data and transfers it to userland."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:389
msgid ""
"See man:g_bio[9] man page for information how the data is passed back and "
"forth in the `bio` structure (note in particular the `bio_parent` and "
"`bio_children` fields and how they are handled)."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:392
msgid ""
"One important feature is: __THERE CAN BE NO SLEEPING IN G_UP AND G_DOWN "
"THREADS__.  This means that none of the following things can be done in "
"those threads (the list is of course not complete, but only informative):"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:394
msgid "Calls to `msleep`() and `tsleep`(), obviously."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:395
msgid ""
"Calls to `g_write_data`() and `g_read_data`(), because these sleep between "
"passing the data to consumers and returning."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:396
msgid "Waiting for I/O."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:397
msgid "Calls to man:malloc[9] and `uma_zalloc`() with `M_WAITOK` flag set"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:398
msgid "sx and other sleepable locks"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:402
msgid ""
"This restriction is here to stop GEOM code clogging the I/O request path, "
"since sleeping is usually not time-bound and there can be no guarantees on "
"how long will it take (there are some other, more technical reasons also).  "
"It also means that there is not much that can be done in those threads; for "
"example, almost any complex thing requires memory allocation.  Fortunately, "
"there is a way out: creating additional kernel threads."
msgstr ""

#. type: Title ===
#: documentation/content/en/articles/geom-class/_index.adoc:404
#, no-wrap
msgid "Kernel Threads for Use in GEOM Code"
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:407
msgid ""
"Kernel threads are created with man:kthread_create[9] function, and they are "
"sort of similar to userland threads in behavior, only they cannot return to "
"caller to signify termination, but must call man:kthread_exit[9]."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:410
msgid ""
"In GEOM code, the usual use of threads is to offload processing of requests "
"from `g_down` thread (the `.start`() function).  These threads look like "
"\"event handlers\": they have a linked list of event associated with them "
"(on which events can be posted by various functions in various threads so it "
"must be protected by a mutex), take the events from the list one by one and "
"process them in a big `switch`() statement."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:415
msgid ""
"The main benefit of using a thread to handle I/O requests is that it can "
"sleep when needed.  Now, this sounds good, but should be carefully thought "
"out.  Sleeping is well and very convenient but can very effectively destroy "
"performance of the geom transformation.  Extremely performance-sensitive "
"classes probably should do all the work in `.start`() function call, taking "
"great care to handle out-of-memory and similar errors."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:419
msgid ""
"The other benefit of having a event-handler thread like that is to serialize "
"all the requests and responses coming from different geom threads into one "
"thread.  This is also very convenient but can be slow.  In most cases, "
"handling of `.done`() requests can be left to the `g_up` thread."
msgstr ""

#. type: Plain text
#: documentation/content/en/articles/geom-class/_index.adoc:422
msgid ""
"Mutexes in FreeBSD kernel (see man:mutex[9]) have one distinction from their "
"more common userland cousins - the code cannot sleep while holding a "
"mutex).  If the code needs to sleep a lot, man:sx[9] locks may be more "
"appropriate.  On the other hand, if you do almost everything in a single "
"thread, you may get away with no mutexes at all."
msgstr ""