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doc: Add new Swap Space section.
* doc/guix.texi (operating-system Reference): Update swap-devices. * doc/guix.texi (Swap Space): Add it. * gnu/system/examples/desktop.tmpl: Add swap-devices example. Signed-off-by: Ludovic Courtès <ludo@gnu.org>
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Josselin Poiret
Ludovic Courtès
parent
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commit
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2 changed files with 116 additions and 38 deletions
147
doc/guix.texi
147
doc/guix.texi
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@ -319,6 +319,7 @@ System Configuration
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* operating-system Reference:: Detail of operating-system declarations.
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* File Systems:: Configuring file system mounts.
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* Mapped Devices:: Block device extra processing.
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* Swap Space:: Backing RAM with disk space.
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* User Accounts:: Specifying user accounts.
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* Keyboard Layout:: How the system interprets key strokes.
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* Locales:: Language and cultural convention settings.
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@ -2515,10 +2516,9 @@ system relative to this path. If you have opted for @file{/boot/efi} as an
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EFI mount point for example, mount it at @file{/mnt/boot/efi} now so it is
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found by @code{guix system init} afterwards.
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Finally, if you plan to use one or more swap partitions (@pxref{Memory
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Concepts, swap space,, libc, The GNU C Library Reference Manual}), make
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sure to initialize them with @command{mkswap}. Assuming you have one
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swap partition on @file{/dev/sda3}, you would run:
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Finally, if you plan to use one or more swap partitions (@pxref{Swap
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Space}), make sure to initialize them with @command{mkswap}. Assuming
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you have one swap partition on @file{/dev/sda3}, you would run:
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@example
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mkswap /dev/sda3
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@ -13987,6 +13987,7 @@ instance to support new system services.
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* operating-system Reference:: Detail of operating-system declarations.
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* File Systems:: Configuring file system mounts.
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* Mapped Devices:: Block device extra processing.
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* Swap Space:: Backing RAM with disk space.
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* User Accounts:: Specifying user accounts.
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* Keyboard Layout:: How the system interprets key strokes.
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* Locales:: Language and cultural convention settings.
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@ -14155,7 +14156,7 @@ configuration, but with a few modifications.
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@cindex encrypted disk
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The configuration for a typical ``desktop'' usage, with an encrypted
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root partition, the X11 display
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root partition, a swap file on the root partition, the X11 display
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server, GNOME and Xfce (users can choose which of these desktop
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environments to use at the log-in screen by pressing @kbd{F1}), network
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management, power management, and more, would look like this:
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@ -14353,38 +14354,9 @@ A list of mapped devices. @xref{Mapped Devices}.
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@item @code{file-systems}
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A list of file systems. @xref{File Systems}.
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@cindex swap devices
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@cindex swap space
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@item @code{swap-devices} (default: @code{'()})
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A list of UUIDs, file system labels, or strings identifying devices or
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files to be used for ``swap
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space'' (@pxref{Memory Concepts,,, libc, The GNU C Library Reference
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Manual}). Here are some examples:
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@table @code
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@item (list (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb"))
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Use the swap partition with the given UUID@. You can learn the UUID of a
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Linux swap partition by running @command{swaplabel @var{device}}, where
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@var{device} is the @file{/dev} file name of that partition.
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@item (list (file-system-label "swap"))
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Use the partition with label @code{swap}. Again, the
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@command{swaplabel} command allows you to view and change the label of a
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Linux swap partition.
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@item (list "/swapfile")
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Use the file @file{/swapfile} as swap space.
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@item (list "/dev/sda3" "/dev/sdb2")
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Use the @file{/dev/sda3} and @file{/dev/sdb2} partitions as swap space.
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We recommend referring to swap devices by UUIDs or labels as shown above
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instead.
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@end table
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It is possible to specify a swap file in a file system on a mapped
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device (under @file{/dev/mapper}), provided that the necessary device
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mapping and file system are also specified. @xref{Mapped Devices} and
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@ref{File Systems}.
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@cindex swap devices
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A list of swap spaces. @xref{Swap Space}.
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@item @code{users} (default: @code{%base-user-accounts})
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@itemx @code{groups} (default: @code{%base-groups})
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@ -14974,7 +14946,8 @@ It is also desirable to encrypt swap space, since swap space may contain
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sensitive data. One way to accomplish that is to use a swap file in a
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file system on a device mapped via LUKS encryption. In this way, the
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swap file is encrypted because the entire device is encrypted.
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@xref{Preparing for Installation,,Disk Partitioning}, for an example.
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@xref{Swap Space}, or @xref{Preparing for Installation,,Disk
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Partitioning}, for an example.
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A RAID device formed of the partitions @file{/dev/sda1} and @file{/dev/sdb1}
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may be declared as follows:
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@ -15006,6 +14979,106 @@ Devices @file{/dev/mapper/vg0-alpha} and @file{/dev/mapper/vg0-beta} can
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then be used as the @code{device} of a @code{file-system} declaration
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(@pxref{File Systems}).
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@node Swap Space
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@section Swap Space
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@cindex swap space
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Swap space, as it is commonly called, is a disk area specifically
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designated for paging: the process in charge of memory management
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(the Linux kernel or Hurd's default pager) can decide that some memory
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pages stored in RAM which belong to a running program but are unused
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should be stored on disk instead. It unloads those from the RAM,
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freeing up precious fast memory, and writes them to the swap space. If
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the program tries to access that very page, the memory management
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process loads it back into memory for the program to use.
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A common misconception about swap is that it is only useful when small
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amounts of RAM are available to the system. However, it should be noted
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that kernels often use all available RAM for disk access caching to make
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I/O faster, and thus paging out unused portions of program memory will
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expand the RAM available for such caching.
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For a more detailed description of how memory is managed from the
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viewpoint of a monolithic kernel, @xref{Memory
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Concepts,,, libc, The GNU C Library Reference Manual}.
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The Linux kernel has support for swap partitions and swap files: the
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former uses a whole disk partition for paging, whereas the second uses a
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file on a file system for that (the file system driver needs to support
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it). On a comparable setup, both have the same performance, so one
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should consider ease of use when deciding between them. Partitions are
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``simpler'' and do not need file system support, but need to be
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allocated at disk formatting time (logical volumes notwithstanding),
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whereas files can be allocated and deallocated at any time.
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Note that swap space is not zeroed on shutdown, so sensitive data (such
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as passwords) may linger on it if it was paged out. As such, you should
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consider having your swap reside on an encrypted device (@pxref{Mapped
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Devices}).
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@deftp {Data Type} swap-space
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Objects of this type represent swap spaces. They contain the following
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members:
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@table @asis
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@item @code{target}
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The device or file to use, either a UUID, a @code{file-system-label} or
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a string, as in the definition of a @code{file-system} (@pxref{File
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Systems}).
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@item @code{dependencies} (default: @code{'()})
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A list of @code{file-system} or @code{mapped-device} objects, upon which
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the availability of the space depends. Note that just like for
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@code{file-system} objects, dependencies which are needed for boot and
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mounted in early userspace are not managed by the Shepherd, and so
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automatically filtered out for you.
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@item @code{priority} (default: @code{#f})
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Only supported by the Linux kernel. Either @code{#f} to disable swap
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priority, or an integer between 0 and 32767. The kernel will first use
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swap spaces of higher priority when paging, and use same priority spaces
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on a round-robin basis. The kernel will use swap spaces without a set
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priority after prioritized spaces, and in the order that they appeared in
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(not round-robin).
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@item @code{discard?} (default: @code{#f})
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Only supported by the Linux kernel. When true, the kernel will notify
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the disk controller of discarded pages, for example with the TRIM
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operation on Solid State Drives.
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@end table
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@end deftp
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Here are some examples:
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@lisp
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(swap-space (target (uuid "4dab5feb-d176-45de-b287-9b0a6e4c01cb")))
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@end lisp
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Use the swap partition with the given UUID@. You can learn the UUID of a
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Linux swap partition by running @command{swaplabel @var{device}}, where
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@var{device} is the @file{/dev} file name of that partition.
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@lisp
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(swap-space
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(target (file-system-label "swap"))
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(dependencies (list lvm-device)))
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@end lisp
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Use the partition with label @code{swap}, which can be found after the
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@var{lvm-device} mapped device has been opened. Again, the
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@command{swaplabel} command allows you to view and change the label of a
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Linux swap partition.
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@lisp
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(swap-space
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(target "/btrfs/swapfile")
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(dependencies (list btrfs-fs)))
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@end lisp
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Use the file @file{/btrfs/swapfile} as swap space, which is present on the
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@var{btrfs-fs} filesystem.
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@node User Accounts
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@section User Accounts
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@ -1,6 +1,6 @@
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;; This is an operating system configuration template
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;; for a "desktop" setup with GNOME and Xfce where the
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;; root partition is encrypted with LUKS.
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;; root partition is encrypted with LUKS, and a swap file.
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(use-modules (gnu) (gnu system nss))
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(use-service-modules desktop xorg)
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@ -42,6 +42,11 @@
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(type "vfat")))
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%base-file-systems))
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;; Specify a swap file for the system, which resides on the
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;; root file system.
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(swap-devices (list (swap-space
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(target "/swapfile"))))
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;; Create user `bob' with `alice' as its initial password.
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(users (cons (user-account
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(name "bob")
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