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.\" ========================================================================
.\"
.IX Title "nbdkit-memory-plugin 1"
.TH nbdkit-memory-plugin 1 2024-05-10 nbdkit-1.38.2 NBDKIT
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.nh
.SH NAME
nbdkit\-memory\-plugin \- nbdkit virtual memory (RAM disk) plugin
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit memory [size=]SIZE [allocator=sparse|malloc|zstd]
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-memory\-plugin\*(C'\fR is a plugin for \fBnbdkit\fR\|(1) which stores a
single disk image in virtual memory, and discards it when nbdkit
exits.  This plugin can be used for testing or where you don't care
about the final content of the disk image.
.PP
All nbdkit clients will see the same disk content, initially all
zeroes.
.PP
By default the disk image is stored in memory using a sparse array.
The allocated parts of the disk image cannot be larger than physical
RAM plus swap, less whatever is being used by the rest of the system.
Other allocators are available, see "ALLOCATORS" below.  All
allocators store the image in memory.  If you want to allocate more
space than this use \fBnbdkit\-file\-plugin\fR\|(1) backed by a temporary
file instead.
.PP
Using the sparse allocator the virtual size can be as large as you
like, up to the maximum supported by nbdkit (2⁶³\-1\ bytes).  This
limit is tested when nbdkit is compiled, and it should work on all
platforms and architectures supported by nbdkit.
.SH EXAMPLES
.IX Header "EXAMPLES"
Create a one gigabyte sparse RAM disk:
.PP
.Vb 1
\& nbdkit memory 1G
.Ve
.PP
If you want to loop mount the above disk, see \fBnbdkit\-loop\fR\|(1).
.PP
Create the largest possible RAM disk:
.PP
.Vb 1
\& nbdkit memory $(( 2**63 \- 1 ))
.Ve
.SH PARAMETERS
.IX Header "PARAMETERS"
.IP [\fBsize=\fR]SIZE 4
.IX Item "[size=]SIZE"
Specify the virtual size of the disk image.
.Sp
This parameter is required.
.Sp
\&\f(CW\*(C`size=\*(C'\fR is a magic config key and may be omitted in most cases.
See "Magic parameters" in \fBnbdkit\fR\|(1).
.IP \fBallocator=sparse\fR 4
.IX Item "allocator=sparse"
.PD 0
.IP \fBallocator=malloc\fR[,\fBmlock=true\fR] 4
.IX Item "allocator=malloc[,mlock=true]"
.IP \fBallocator=zstd\fR 4
.IX Item "allocator=zstd"
.PD
(nbdkit ≥ 1.22)
.Sp
Select the backend allocation strategy.  See "ALLOCATORS" below.
The default is sparse.
.SH NOTES
.IX Header "NOTES"
.SS "Preloading small amounts of data"
.IX Subsection "Preloading small amounts of data"
If you want an in-memory disk image preinitialized with a small amount
of data specified on the command line, look at
\&\fBnbdkit\-data\-plugin\fR\|(1) instead.  Note by "small" this does not mean
that the virtual disk image must be small, but that the amount of data
initially stored sparsely is small enough to specify on the command
line.
.SS "Preloading large amounts of data"
.IX Subsection "Preloading large amounts of data"
If you want to preload a large amount of data (eg. a disk image) into
the memory plugin, use \fBqemu\-img\fR\|(1) or \fBnbdcopy\fR\|(1):
.PP
.Vb 2
\& $ rm \-f pid
\& $ nbdkit \-P pid memory 10G
.Ve
.PP
Wait for nbdkit to become ready to accept connections:
.PP
.Vb 1
\& $ while [ ! \-f pid ]; do sleep 1; done
.Ve
.PP
Preload Fedora disk image using qemu-img:
.PP
.Vb 2
\& $ virt\-builder fedora\-28 \-\-size=10G
\& $ qemu\-img convert \-p \-n fedora\-28.img nbd:localhost:10809
.Ve
.PP
If you have libnbd ≥ 1.4, you can use \fBnbdcopy\fR\|(1) as an
alternative:
.PP
.Vb 1
\& $ nbdcopy \-p fedora\-28.img nbd://localhost
.Ve
.SH ALLOCATORS
.IX Header "ALLOCATORS"
Since nbdkit ≥ 1.22 several allocation strategies are available
using the \f(CW\*(C`allocator\*(C'\fR parameter.
.IP \fBallocator=sparse\fR 4
.IX Item "allocator=sparse"
The disk image is stored in memory using a sparse array.  The sparse
array uses a simple two level page table with a fixed page size.  The
allocated parts of the disk image cannot be larger than physical RAM
plus swap, less whatever is being used by the rest of the system.  The
aim of the sparse array implementation is to support extremely large
images for testing, although it won't necessarily be efficient for
that use case.  However it should also be reasonably efficient for
normal disk sizes.
.Sp
The virtual size of the disk can be as large as you like, up to the
maximum supported by nbdkit (2⁶³\-1\ bytes).
.Sp
This is the default, and was the only allocator available before
nbdkit\ 1.22.
.IP \fBallocator=malloc\fR 4
.IX Item "allocator=malloc"
.PD 0
.IP \fBallocator=malloc,mlock=true\fR 4
.IX Item "allocator=malloc,mlock=true"
.PD
The disk image is stored directly in memory allocated using
\&\fBmalloc\fR\|(3) on the heap.  No sparseness is possible: you must have
enough memory for the whole disk.  Very large virtual sizes will
usually fail.  However this can be faster because the implementation
is simpler and the locking strategy allows more concurrency.
.Sp
If \f(CW\*(C`mlock=true\*(C'\fR is added then additionally the array is locked into
RAM using \fBmlock\fR\|(2) (so it should never be swapped out).  This
usually requires you to adjust the \fBulimit\fR\|(1) associated with the
process and on some operating systems may require you to run nbdkit as
root.  (See also the \fBnbdkit\fR\|(1) \fI\-\-swap\fR option).
.Sp
The \f(CW\*(C`mlock=true\*(C'\fR feature is only supported on some platforms.  Use
\&\f(CW\*(C`nbdkit\ memory\ \-\-dump\-plugin\*(C'\fR and check that the output contains
\&\f(CW\*(C`mlock=yes\*(C'\fR.
.IP \fBallocator=zstd\fR 4
.IX Item "allocator=zstd"
The disk image is stored in a sparse array where each page is
compressed using zstd compression.
Assuming a typical 2:1 compression ratio, this allows you to store
twice as much real data as \f(CW\*(C`allocator=sparse\*(C'\fR, with the trade-off
that the plugin is slightly slower because it has to compress and
decompress each page.  Aside from compression, the implementation of
this allocator is similar to \f(CW\*(C`allocator=sparse\*(C'\fR, so in other respects
(such as supporting huge virtual disk sizes) it is the same.
.Sp
This allocator is only supported if nbdkit was compiled with zstd
support.  Use \f(CW\*(C`nbdkit\ memory\ \-\-dump\-plugin\*(C'\fR and check that the
output contains \f(CW\*(C`zstd=yes\*(C'\fR.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$plugindir\fR\fI/nbdkit\-memory\-plugin.so\fR 4
.el .IP \fR\f(CI$plugindir\fR\fI/nbdkit\-memory\-plugin.so\fR 4
.IX Item "$plugindir/nbdkit-memory-plugin.so"
The plugin.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$plugindir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-memory\-plugin\*(C'\fR first appeared in nbdkit 1.2.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-plugin\fR\|(3),
\&\fBnbdkit\-loop\fR\|(1),
\&\fBnbdkit\-data\-plugin\fR\|(1),
\&\fBnbdkit\-file\-plugin\fR\|(1),
\&\fBnbdkit\-info\-plugin\fR\|(1),
\&\fBnbdkit\-tmpdisk\-plugin\fR\|(1),
\&\fBmlock\fR\|(2),
\&\fBmalloc\fR\|(3),
\&\fBqemu\-img\fR\|(1),
\&\fBnbdcopy\fR\|(1).
.SH AUTHORS
.IX Header "AUTHORS"
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP \(bu 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP \(bu 4
Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
.IP \(bu 4
Neither the name of Red Hat nor the names of its contributors may be
used to endorse or promote products derived from this software without
specific prior written permission.
.PP
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.