^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) ==========================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) Using the RAM disk block device with Linux
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3) ==========================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5) .. Contents:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) 1) Overview
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) 2) Kernel Command Line Parameters
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) 3) Using "rdev"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10) 4) An Example of Creating a Compressed RAM Disk
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) 1) Overview
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14) -----------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) The RAM disk driver is a way to use main system memory as a block device. It
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) is required for initrd, an initial filesystem used if you need to load modules
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) also be used for a temporary filesystem for crypto work, since the contents
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) are erased on reboot.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22) The RAM disk dynamically grows as more space is required. It does this by using
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) RAM from the buffer cache. The driver marks the buffers it is using as dirty
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) so that the VM subsystem does not try to reclaim them later.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) to support an unlimited number of RAM disks (at your own risk). Just change
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) and (re)build the kernel.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) directory. RAM disks are all major number 1, and start with minor number 0
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35) The new RAM disk also has the ability to load compressed RAM disk images,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) allowing one to squeeze more programs onto an average installation or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) rescue floppy disk.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40) 2) Parameters
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) ---------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43) 2a) Kernel Command Line Parameters
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45) ramdisk_size=N
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46) Size of the ramdisk.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) This parameter tells the RAM disk driver to set up RAM disks of N k size. The
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49) default is 4096 (4 MB).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51) 2b) Module parameters
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53) rd_nr
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) /dev/ramX devices created.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) max_part
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57) Maximum partition number.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59) rd_size
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) See ramdisk_size.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62) 3) Using "rdev"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63) ---------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65) "rdev" is an obsolete, deprecated, antiquated utility that could be used
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) to set the boot device in a Linux kernel image.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68) Instead of using rdev, just place the boot device information on the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) kernel command line and pass it to the kernel from the bootloader.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71) You can also pass arguments to the kernel by setting FDARGS in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73) arch/x86/boot/Makefile.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75) Some of the kernel command line boot options that may apply here are::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) ramdisk_start=N
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78) ramdisk_size=M
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80) If you make a boot disk that has LILO, then for the above, you would use::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) append = "ramdisk_start=N ramdisk_size=M"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) 4) An Example of Creating a Compressed RAM Disk
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85) -----------------------------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87) To create a RAM disk image, you will need a spare block device to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) construct it on. This can be the RAM disk device itself, or an
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) unused disk partition (such as an unmounted swap partition). For this
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90) example, we will use the RAM disk device, "/dev/ram0".
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) Note: This technique should not be done on a machine with less than 8 MB
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) of RAM. If using a spare disk partition instead of /dev/ram0, then this
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) restriction does not apply.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) a) Decide on the RAM disk size that you want. Say 2 MB for this example.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97) Create it by writing to the RAM disk device. (This step is not currently
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98) required, but may be in the future.) It is wise to zero out the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) area (esp. for disks) so that maximal compression is achieved for
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100) the unused blocks of the image that you are about to create::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) b) Make a filesystem on it. Say ext2fs for this example::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) mke2fs -vm0 /dev/ram0 2048
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108) c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109) and unmount it again.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) d) Compress the contents of the RAM disk. The level of compression
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) will be approximately 50% of the space used by the files. Unused
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) space on the RAM disk will compress to almost nothing::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) e) Put the kernel onto the floppy::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) dd if=zImage of=/dev/fd0 bs=1k
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121) f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122) that is slightly larger than the kernel, so that you can put another
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) (possibly larger) kernel onto the same floppy later without overlapping
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) the RAM disk image. An offset of 400 kB for kernels about 350 kB in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125) size would be reasonable. Make sure offset+size of ram_image.gz is
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126) not larger than the total space on your floppy (usually 1440 kB)::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 128) dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 129)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 130) g) Make sure that you have already specified the boot information in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 131) FDARGS and FDINITRD or that you use a bootloader to pass kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 132) command line boot options to the kernel.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 133)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 134) That is it. You now have your boot/root compressed RAM disk floppy. Some
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 135) users may wish to combine steps (d) and (f) by using a pipe.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 136)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 137)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 138) Paul Gortmaker 12/95
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 139)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 140) Changelog:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 141) ----------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 142)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 143) SEPT-2020 :
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 144)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 145) Removed usage of "rdev"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 146)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 147) 10-22-04 :
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 148) Updated to reflect changes in command line options, remove
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 149) obsolete references, general cleanup.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 150) James Nelson (james4765@gmail.com)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 151)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 152) 12-95 :
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 153) Original Document
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
3 Commits
0 Branches
0 Tags