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8.8. Configurar el carregador d'arrencada

Probablement ja és funcional, però sempre és bo saber com configurar i instal·lar el carregador d'arrencada en cas que desaparegui del «Master Boot Record». Això pot ocórrer després de la instal·lació d'un altre sistema operatiu, com ara Windows. La següent informació també us pot ajudar a modificar la configuració del gestor d'arrencada si calgués.

TORNAR A LES BASES «Master boot record»

El «Master Boot Record» (MBR, o Registre Mestre d'Arrencada) ocupa els primers 512 bytes del primer disc dur, i és el primer que es carrega per part de la BIOS per lliurar el control a un programa capaç d'arrencar el sistema operatiu desitjat. En general, un carregador d'arrencada s'instal·la al MBR, eliminant el seu contingut anterior.

8.8.1. Identificació dels discos

CULTURA udev i /dev/

El directori /dev/ alberga tradicionalment els anomenats fitxers "especials", destinats a representar els perifèrics del sistema (vegeu la barra lateral TORNAR A LES BASES Permisos d'accés a dispositius). Antigament solia contenir tots els fitxers especials que podien ser utilitzats. Aquest enfocament tenia una sèrie de inconvenients entre els quals el fet que restringia el nombre de dispositius que es podia utilitzar (a causa de ser una llista prederminada de noms), i que era impossible saber quins fitxers especials eren realment útils.
Avui dia la gestió d'arxius especials és totalment dinàmica i encaixa millor amb la naturalesa dels dispositius d'ordinador que es poden utilitzar en calent. El nucli coopera amb udev (Secció 9.11.3, «Com funciona udev») per crear-los i eliminar-los segons sigui necessari quan els dispositius corresponents apareixen i desapareixen. Per aquesta raó, /dev/ no necessita ser persistent i és, per tant, un sistema de fitxers basat en la RAM que comença buit i només conté les entrades rellevants.
The kernel communicates lots of information about any newly added device and hands out a pair of major/minor numbers to identify it. With this udevd can create the special file under the name and with the permissions that it wants. It can also create aliases and perform additional actions (such as initialization or registration tasks). udevd's behavior is driven by a large set of (customizable) rules usually found in /lib/udev/rules.d/ and /etc/udev/rules.d/.
Amb noms assignats dinàmicament, podeu mantenir el mateix nom per a un dispositiu donat, independentment del connector utilitzat o l'ordre de connexió, fet que és especialment útil quan utilitzeu diversos perifèrics USB. La primera partició en el primer disc dur es pot anomenar /dev/sda1 per a la compatibilitat cap enrere, o /dev/root-partition si ho preferiu, o fins i tot ambdues alhora, ja que udevd es pot configurar per crear automàticament un enllaç simbòlic.
In ancient times, some kernel modules did automatically load when you tried to access the corresponding device file. This is no longer the case, and the peripheral's special file no longer exists prior to loading the module; this is no big deal, since most modules are loaded on boot thanks to automatic hardware detection. But for undetectable peripherals (such as very old disk drives or PS/2 mice), this doesn't work. Consider adding the modules, floppy, psmouse and mousedev to /etc/modules or /etc/modules-load.d/ in order to force loading them on boot.
La configuració del carregador d'arrencada ha d'identificar els diferents discos durs i les seves particions. Linux utilitza fitxers especials de “bloc” ubicats al directori /dev/ per a aquest propòsit. Des de Debian Squeeze, la nomenclatura per a discos durs ha estat unificada pel nucli Linux, i tots els discos durs (IDE/PATA, SATA, SCSI, USB, IEEE 1394) ara estan representades per /dev/sd*.
Cada partició està representada pel seu número al disc on resideix: per exemple, /dev/sda1 és la primera partició del primer disc, i /dev/sdb3 és la tercera partició del segon disc.
L'arquitectura de PC (o “i386”, incloent el seu cosí més jove “amd64”) s'ha limitat durant molt de temps a utilitzar el format de taula de particions “MS-DOS”, que només permet quatre particions “primàries” per disc. Per anar més enllà de la limitació d'aquest esquema, una de les particions ha de ser creada com a partició "estesa", i llavors pot contenir particions “secundàries” addicionals. Aquestes particions secundàries estan numerades a partir de 5. Per tant, la primera partició secundària podria ser /dev/sda5, seguida de /dev/sda6, etc.
Una altra restricció del format de taula de particions MS-DOS és que només permet discos de fins a 2 TiB de mida, que comença a ser un problema real amb els discs recents.
A new partition table format called GPT (GUID Partition Table) loosens these constraints on the number of partitions (it allows up to 128 partitions when using standard settings) and on the size of the disks (up to 8 ZiB, which is more than 8 billion terabytes). If you intend to create many physical partitions on the same disk, you should therefore ensure that you are creating the partition table in the GPT format when partitioning your disk.

QUICK LOOK The Globally Unique Identifier (GUID)

Using GPT the partition type is represented as a 16 bytes long universally unique identifier, also known as Globally Unique Identifier (GUID), which is part of the UEFI standard (see NOTA UEFI, un reemplaçament modern per la BIOS). Compared to the hex code used to determine the partition type in the MS-DOS table format, you will have a hard time trying to remember these identifiers. Fortunately there are lists with all the IDs which you can use to create partition recipes to use with sfdisk or for preseeding the Debian installer.
→ https://en.wikipedia.org/wiki/GUID_Partition_Table#Partition_type_GUIDs
It is not always easy to remember what disk is connected to which SATA controller, or in third position in the SCSI chain, especially since the naming of hotplugged hard drives (which includes among others most SATA disks and external disks) can change from one boot to another. Fortunately, udev creates, in addition to /dev/sd*, symbolic links with a fixed name, which you could then use if you wished to identify a hard drive in a non-ambiguous manner. These symbolic links are stored in /dev/disk/by-id/. On a machine with two physical disks, for example, one could find the following: 
mirexpress:/dev/disk/by-id# ls -l
total 0
lrwxrwxrwx 1 root root  9 23 jul. 08:58 ata-STM3500418AS_9VM3L3KP -> ../../sda
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-STM3500418AS_9VM3L3KP-part1 -> ../../sda1
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-STM3500418AS_9VM3L3KP-part2 -> ../../sda2
[...]
lrwxrwxrwx 1 root root  9 23 jul. 08:58 ata-WDC_WD5001AALS-00L3B2_WD-WCAT00241697 -> ../../sdb
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-WDC_WD5001AALS-00L3B2_WD-WCAT00241697-part1 -> ../../sdb1
lrwxrwxrwx 1 root root 10 23 jul. 08:58 ata-WDC_WD5001AALS-00L3B2_WD-WCAT00241697-part2 -> ../../sdb2
[...]
lrwxrwxrwx 1 root root  9 23 jul. 08:58 scsi-SATA_STM3500418AS_9VM3L3KP -> ../../sda
lrwxrwxrwx 1 root root 10 23 jul. 08:58 scsi-SATA_STM3500418AS_9VM3L3KP-part1 -> ../../sda1
lrwxrwxrwx 1 root root 10 23 jul. 08:58 scsi-SATA_STM3500418AS_9VM3L3KP-part2 -> ../../sda2
[...]
lrwxrwxrwx 1 root root  9 23 jul. 08:58 scsi-SATA_WDC_WD5001AALS-_WD-WCAT00241697 -> ../../sdb
lrwxrwxrwx 1 root root 10 23 jul. 08:58 scsi-SATA_WDC_WD5001AALS-_WD-WCAT00241697-part1 -> ../../sdb1
lrwxrwxrwx 1 root root 10 23 jul. 08:58 scsi-SATA_WDC_WD5001AALS-_WD-WCAT00241697-part2 -> ../../sdb2
[...]
lrwxrwxrwx 1 root root  9 23 jul. 16:48 usb-LaCie_iamaKey_3ed00e26ccc11a-0:0 -> ../../sdc
lrwxrwxrwx 1 root root 10 23 jul. 16:48 usb-LaCie_iamaKey_3ed00e26ccc11a-0:0-part1 -> ../../sdc1
lrwxrwxrwx 1 root root 10 23 jul. 16:48 usb-LaCie_iamaKey_3ed00e26ccc11a-0:0-part2 -> ../../sdc2
[...]
lrwxrwxrwx 1 root root  9 23 jul. 08:58 wwn-0x5000c50015c4842f -> ../../sda
lrwxrwxrwx 1 root root 10 23 jul. 08:58 wwn-0x5000c50015c4842f-part1 -> ../../sda1
[...]
mirexpress:/dev/disk/by-id#
Note that some disks are listed several times (because they behave simultaneously as ATA disks and SCSI disks), but the relevant information is mainly in the model and serial numbers of the disks, from which you can find the peripheral file. While the links in /dev/disk/by-id/ are created using the device' serial number and physical path, there are more convenience links in e.g. /dev/disk/by-label/ (based on given labels), /dev/disk/by-uuid/ (based on unique identifiers, which can change when reformatting a device using mkfs.* or mkswap), /dev/disk/by-path/ (based on shortest physical path), and /dev/disk/by-partlabel/ and /dev/disk/by-partuuid/ (only partitions with GPT labels and their unique identifiers). If you use these links, e.g. in /etc/fstab, always prefer unique identifiers over labels. You can also obtain and change this information for each partition or device using the lsblk and blkid commands.
Els fitxers de configuració d'exemple donats en les següents seccions es basen en la mateixa configuració: un sol disc SATA, on la primera partició és una instal·lació antiga de Windows i la segona conté Debian GNU/Linux.

ALTERNATIVE The LILO bootloader

LILO («Linux Loader» o carregador Linux) és el carregador d'arrencada més antic — sòlid però rústic. Escriu l'adreça física del nucli per arrencar al MBR, per la qual cosa cada actualització de LILO (o del seu fitxer de configuració) ha de ser seguida per l'execució l'ordre lilo. Si s'oblida fer-ho, el sistema no podrà arrencar si el nucli antic s'ha eliminat o substituït perquè el nou no estarà en la mateixa ubicació del disc.
El fitxer de configuració de LILO és /etc/lilo.conf; un fitxer senzill d'una configuració estàndard s'il·lustra a l'exemple següent.

Exemple 8.4. Fitxer de configuració de LILO

# El disc on el LILO s'ha d'instal·lar
# Indicant un disc i no una partició
# feu que el LILO s'instal·li a l'MBR.
boot=/dev/sda
# la partició que conté Debian
root=/dev/sda2
# l'ítem que es carregarà per defecte
default=Linux

# la imatge de nucli més recent
image=/vmlinuz
  label=Linux
  initrd=/initrd.img
  read-only

# nucli antic (per si el nucli recentment instal·lat no funciona)
image=/vmlinuz.old
  label=LinuxOLD
  initrd=/initrd.img.old
  read-only
  optional

# només per a l'arrencada dual Linux/Windows 
other=/dev/sda1
  label=Windows
As per request of the maintainer and upstream author of lilo, the bootloader has been removed from Debian 11 Bullseye and it is unlikely to make a comeback.

8.8.2. Configuració de GRUB 2

GRUB (GRand Unified Bootloader) is more recent. It is not necessary to invoke it after each update of the kernel; GRUB knows how to read the filesystems and find the position of the kernel on the disk by itself. To install it on the MBR of the first disk, simply type grub-install /dev/sda. This will overwrite the MBR, so be careful not to overwrite the wrong location. While it is also possible to install GRUB into a partition boot record, beware that it is usually a mistake and doing grub-install /dev/sda1 has not the same meaning as grub-install /dev/sda.

NOTA Noms de disc per al GRUB

GRUB can only identify hard drives based on information provided by the BIOS. (hd0) corresponds to the first disk thus detected, (hd1) the second, etc. In most cases, this order corresponds exactly to the usual order of disks under Linux, but problems can occur when you associate SCSI and IDE disks. GRUB used to store the correspondences that it detected in the file /boot/grub/device.map. Nowadays it avoids this problem using universally unique identifier (UUIDs) or file system labels when generating grub.cfg. However, the device map file is not obsolete yet, since it can be used to override when the current environment is different from the one on boot. If you find errors there (because you know that your BIOS detects drives in a different order), correct them manually and run grub-install again. grub-mkdevicemap can help creating a device.map file from which to start.
Les particions també tenen un nom específic amb GRUB. Quan utilitzeu particions “clàssiques” en format MS-DOS, la primera partició del primer disc està etiquetada com a (hd0,msdos1), la segona (hd0,msdos2), etc.
GRUB 2 configuration is stored in /boot/grub/grub.cfg, but this file (in Debian) is generated from others. Be careful not to modify it by hand, since such local modifications will be lost the next time update-grub is run (which may occur upon update of various packages). The most common modifications of the /boot/grub/grub.cfg file (to add command line parameters to the kernel or change the duration that the menu is displayed, for example) are made through the variables in /etc/default/grub. To add entries to the menu, you can either create a /boot/grub/custom.cfg file or modify the /etc/grub.d/40_custom file. For more complex configurations, you can modify other files in /etc/grub.d, or add to them; these scripts should return configuration snippets, possibly by making use of external programs. These scripts are the ones that will update the list of kernels to boot: 10_linux takes into consideration the installed Linux kernels; 20_linux_xen takes into account Xen virtual systems, and 30_os-prober adds other existing operating systems (Windows, OS X, Hurd), kernel images, and BIOS/EFI access options to the menu.

CAUTION error: symbol `grub_*' not found

One of the most often reported issues when using GRUB is that users get an error like error symbol `grub_calloc' not found and they are unable to boot the system anymore. Most of the time this error is caused by installing an updated version of GRUB that causes new modules in /boot/grub to be incompatible with old core images in the boot sector that your firmware jumps to when booting your machine. This happens on systems that are configured to run grub-install to a target device that is not actually the one that the firmware uses to boot your system (e.g. after replacing disks or moving them around - one can see the issue when running dpkg-reconfigure grub-pc showing the wrong target device). With the release of Debian 11 Bullseye (the change was also populated to Buster) the update process now checks for this issue and exits with an error in case it finds such a constellation.
→ https://bugs.debian.org/bug=966575#95
Setting up grub-pc (2.02+dfsg1-20+deb10u4) ...
/dev/disk/by-id/[..] does not exist, so cannot grub-install to it!
You must correct your GRUB install devices before proceeding:

  DEBIAN_FRONTEND=dialog dpkg --configure grub-pc
  dpkg --configure -a
dpkg: error processing package grub-pc (--configure):
 installed grub-pc package post-installation script subprocess returned error exit status 1
To fix the issue and continue the upgrade procedure, just follow the advice and run the commands given by the error message as root.

8.8.3. Using GRUB with EFI and Secure Boot

Using GRUB to boot either a traditional BIOS system (legacy or UEFI-CSM) or a UEFI system is quite different. Fortunately the user doesn't need to know the differences because Debian provides different packages for each purpose and the installer automatically cares about which one(s) to choose. The grub-pc package is chosen for legacy systems, where GRUB is installed into the MBR, while UEFI systems require grub-efi-arch, where GRUB is installed into the EFI System Partition (ESP). The latter requires a GTP partition table as well as an EFI partition.
→ https://wiki.debian.org/Grub2#UEFI_vs_BIOS_boot
To switch an existing system (supporting UEFI) from legacy to UEFI boot mode not only requires to switch the GRUB packages on the system, but also to adjust the partition table and the to create an EFI partition (probably including resizing existing partitions to create the necessary free space). It is therefore quite an elaborate process and we cannot cover it here. Fortunately, there are some manuals by bloggers describing the necessary procedures.
If you are using a system with “Secure Boot“ enabled and have installed shim-signed (see sidebar CULTURA Arrencada segura i el gestor d'arrencada «shim»), you must also install grub-efi-arch-signed. This package is not pulled in automatically, only if the installation of recommended package has been enabled.