Ubuntu Manpage: nbdkit-memory-plugin - nbdkit virtual memory (RAM disk) plugin

Provided by: nbdkit_1.24.1-2ubuntu4_amd64

NAME

       nbdkit-memory-plugin - nbdkit virtual memory (RAM disk) plugin

SYNOPSIS

        nbdkit memory [size=]SIZE [allocator=sparse|...]

DESCRIPTION

       "nbdkit-memory-plugin" is a plugin for nbdkit(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.

       All nbdkit clients will see the same disk content, initially all zeroes.

       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 nbdkit-file-plugin(1) backed by a temporary file instead.

       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.

EXAMPLES

       Create a one gigabyte sparse RAM disk:

        nbdkit memory 1G

       If you want to loop mount the above disk, see nbdkit-loop(1).

       Create the largest possible RAM disk:

        nbdkit memory $(( 2**63 - 1 ))

PARAMETERS

       [size=]SIZE
           Specify the virtual size of the disk image.

           This parameter is required.

           "size=" is a magic config key and may be omitted in most cases.  See "Magic
           parameters" in nbdkit(1).

       allocator=sparse
       allocator=malloc[,mlock=true]
       allocator=zstd
           (nbdkit ≥ 1.22)

           Select the backend allocation strategy.  See "ALLOCATORS" below.  The default is
           sparse.

NOTES

   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 nbdkit-data-plugin(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.

   Preloading large amounts of data
       If you want to preload a large amount of data (eg. a disk image) into the memory plugin,
       use qemu-img(1) or nbdcopy(1):

        $ rm -f pid
        $ nbdkit -P pid memory 10G

       Wait for nbdkit to become ready to accept connections:

        $ while [ ! -f pid ]; do sleep 1; done

       Preload Fedora disk image using qemu-img:

        $ virt-builder fedora-28 --size=10G
        $ qemu-img convert -p -n fedora-28.img nbd:localhost:10809

       If you have libnbd ≥ 1.4, you can use nbdcopy(1) as an alternative:

        $ nbdcopy -p fedora-28.img nbd://localhost

ALLOCATORS

Since nbdkit ≥ 1.22 several allocation strategies are available using the "allocator"
parameter.

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.

The virtual size of the disk can be as large as you like, up to the maximum supported
by nbdkit (2⁶³-1 bytes).

This is the default, and was the only allocator available before nbdkit 1.22.

allocator=malloc
allocator=malloc,mlock=true
The disk image is stored directly in memory allocated using malloc(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.

If "mlock=true" is added then additionally the array is locked into RAM using mlock(2)
(so it should never be swapped out). This usually requires you to adjust the
ulimit(1) associated with the process and on some operating systems may require you to
run nbdkit as root. (See also the nbdkit(1) --swap option).

The "mlock=true" feature is only supported on some platforms. Use
"nbdkit memory --dump-plugin" and check that the output contains "mlock=yes".

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 "allocator=sparse", 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 "allocator=sparse", so
in other respects (such as supporting huge virtual disk sizes) it is the same.

This allocator is only supported if nbdkit was compiled with zstd support. Use
"nbdkit memory --dump-plugin" and check that the output contains "zstd=yes".

FILES

       $plugindir/nbdkit-memory-plugin.so
           The plugin.

           Use "nbdkit --dump-config" to find the location of $plugindir.

VERSION

       "nbdkit-memory-plugin" first appeared in nbdkit 1.2.

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2017-2020 Red Hat Inc.

LICENSE

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