119 lines
2.5 KiB
Groff
119 lines
2.5 KiB
Groff
.\" ----------------------------------------------------------------------------
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.\" "THE BEER-WARE LICENSE" (Revision 42):
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.\" <phk@FreeBSD.ORG> wrote this file. As long as you retain this notice you
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.\" can do whatever you want with this stuff. If we meet some day, and you think
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.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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.\" ----------------------------------------------------------------------------
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.\"
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.\" $FreeBSD$
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.\"
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.Dd July 4, 2000
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.Dt MD 4
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.Os
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.Sh NAME
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.Nm md
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.Nd memory disk
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.Sh SYNOPSIS
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.Cd device md
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.Pp
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To specify the default maximum size of
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.Xr malloc 9
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backed disks:
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.Cd options MD_NSECT=20000
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.Sh DESCRIPTION
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The
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.Nm
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driver provides support for two kinds of memory backed virtual disks:
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.Xr malloc 9
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backed
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and pre-loaded module backed.
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.Pp
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The
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.Xr malloc 9
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backed
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disks
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are severely limited in size by constraints imposed by
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.Xr malloc 9 .
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The limit imposed depends on the amount of physical RAM in the machine,
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and covers all
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.Xr malloc 9
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backed instances of
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.Nm
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devices.
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.Pp
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A sector filled with identical bytes does not consume storage,
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and therefore the storage allocated by a
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.Nm
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disk can be freed with a command such as this:
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.Bd -literal -offset indent
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dd if=/dev/zero of=/dev/md567
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.Ed
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.Pp
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The default maximum size of a
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.Nm
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disk
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backed by
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.Xr malloc 9
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is 20,000 sectors of 512 bytes each.
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This can be changed
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with the kernel option
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.Dv MD_NSECT .
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.Pp
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At boot time the
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.Nm
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driver will search for pre-loaded modules of type
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.Sq md_image
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and instantiate a
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.Nm
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device for each of these modules.
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The type
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.Sq mfs_root
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is also allowed for backward compatibility.
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These devices are backed by the RAM reserved by the
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.Xr loader 8 ,
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and as such not limited by the
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.Xr malloc 9
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size constraints.
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However, this also means that the storage cannot be released.
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For the same reason,
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the
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.Dq identical byte
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detection is not applicable.
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.Pp
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The
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.Nm
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driver uses the
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.Dq almost-clone
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convention,
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whereby opening device number N creates device instance number N+1.
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.Pp
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The
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.Xr vmstat 8
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utility can be used to monitor memory usage by
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.Xr malloc 9
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backed
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.Nm
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disks,
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but not those backed by pre-loaded modules.
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.Sh SEE ALSO
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.Xr disklabel 5 ,
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.Xr disklabel 8 ,
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.Xr fdisk 8 ,
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.Xr loader 8 ,
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.Xr newfs 8 ,
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.Xr vmstat 8
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.Sh HISTORY
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The
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.Nm
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driver first appeared in
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.Fx 4.0
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as a cleaner replacement
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for the MFS functionality previously used in PicoBSD
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and in the FreeBSD installation process.
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.Sh AUTHORS
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The
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.Nm
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driver was written by
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.An Poul-Henning Kamp
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.Aq phk@FreeBSD.org .
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