RAM disk: Difference between revisions
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Taking some RAM and making it look like a disk, probably with a POSIX-style filesystem interface. | |||
This is used for temporary space, because it is much faster than doing the same on platter disk, | |||
and still a decent amount faster than SSD (and doesn't wear such an SSD) | |||
==Linux== | ==Linux== | ||
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===tmpfs and ramfs; shm and /run=== | ===tmpfs and ramfs; shm and /run=== | ||
If you want to ruse RAM as a disk, the immediate options in linux are: | |||
* '''ramfs''' is actually very thin wrapper exposing the RAM cacheing system as usable space. | * '''ramfs''' is actually very thin wrapper exposing the RAM cacheing system as usable space. | ||
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* size: | * size: | ||
:: neither consumes RAM until you store something | :: neither consumes RAM until you store something | ||
:: ramfs will grow as far as it can | :: ramfs will grow as far as it can | ||
:: tmpfs will report an error when it hits its limit (will act like a full disk) | :: tmpfs will report an error when it hits its limit (will act like a full disk) | ||
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* swap: | * swap: | ||
:: ramfs is not eligible for swap | :: ramfs is not eligible for swap | ||
:: tmpfs is eligible for swap | :: tmpfs is eligible for swap | ||
While ramfs is the sometimes-lower-latency option | |||
int that there can be no swapping via possibly-slow IO, | |||
it also means it is up to ''every'' app that uses it to to behave; "will not swap" combined with " can always grow" means you can very efficiently run yourself out of free RAM, cause [[trashing]] and system instability. | |||
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This is a bootstrapping thing | This is a specific use - a bootstrapping thing: the purpose of linux's initramfs (previously initrd) is to mount the root filesystem. | ||
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In some cases it needs to do very little (e.g. mounting a single local disk by device path, containing little more than a basic filesystem driver), and can be nearly empty. | In some cases it needs to do very little (e.g. mounting a single local disk by device path, containing little more than a basic filesystem driver), and can be nearly empty. | ||
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See also: | See also: | ||
Latest revision as of 16:46, 22 April 2024
| The lower-level parts of computers
General: Computer power consumption · Computer noises Memory: Some understanding of memory hardware · CPU cache · Flash memory · Virtual memory · Memory mapped IO and files · RAM disk · Memory limits on 32-bit and 64-bit machines Related: Network wiring notes - Power over Ethernet · 19" rack sizes Unsorted: GPU, GPGPU, OpenCL, CUDA notes · Computer booting
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Taking some RAM and making it look like a disk, probably with a POSIX-style filesystem interface.
This is used for temporary space, because it is much faster than doing the same on platter disk, and still a decent amount faster than SSD (and doesn't wear such an SSD)
Linux
tmpfs and ramfs; shm and /run
If you want to ruse RAM as a disk, the immediate options in linux are:
- ramfs is actually very thin wrapper exposing the RAM cacheing system as usable space.
- tmpfs, based on ramfs, adds a size limit and swappability
- (so acts much more like the classical "allocates a fixed block of RAM" style ram disk)
- and is a bunch safer due to the size limit - though still, pay attention to giving it no more than a good portion of typically-free RAM
The main differences:
- size:
- neither consumes RAM until you store something
- ramfs will grow as far as it can
- tmpfs will report an error when it hits its limit (will act like a full disk)
- df:
- ramfs adds to the 'cached' figure in free/top
- tmpfs shows as a drive in e.g. df
- swap:
- ramfs is not eligible for swap
- tmpfs is eligible for swap
While ramfs is the sometimes-lower-latency option int that there can be no swapping via possibly-slow IO, it also means it is up to every app that uses it to to behave; "will not swap" combined with " can always grow" means you can very efficiently run yourself out of free RAM, cause trashing and system instability.
- Uses
Linuxes keep various transient runtime information in ram disks, largely because there's no point contending for for disk for something you won't ever care to keep.
This includes /var/lock (locks), /var/run (runtime state), /dev/shm (modest data passing), and others that can be distro-specific.
This is usually backed by tmpfs.
Some distros decided to migrate (for now symlink) this stuff to /run so a single mount covers them all - see e.g. [1]
zram
In the linux kernel since 3.2(verify)
zram (previously compcache) provides block devices that are backed by memory and transparently compressed (lzo, lz4, more?).
Uses up to a predefined uncompressed maximum size.
(TODO: figure out whether it's eligible for swap)
This can make sense for
- scratch space
- certain caches, e.g. ZFS L2ARC on zram can make sense
- backing compressed swap, effectively increasing the amount of available RAM when things are swapped out, without using disk to do it
- ...but these days you probably want to use zswap for that
See also:
zswap
Provides swap pages from RAM, rather than from a block device.
Effectively an layer between RAM and disk swap: if the zswap pool is full, or RAM is exhausted, zswap moves pages to disk swap in an LRU style.
Difference between zswap and zram-which-you-happen-to-use-to-back-swapspace
- zram-for-swap is a few more commands
- zram-used-for-swap is separate from existing swap
- so zswap is cleverer when there is disk swap
- and zram often works better when there won't be other swap configured
See also:
- https://wiki.archlinux.org/index.php/Zswap#Compression_algorithm
- https://en.wikipedia.org/wiki/Zswap
initrd, initramfs
This is a specific use - a bootstrapping thing: the purpose of linux's initramfs (previously initrd) is to mount the root filesystem.
initramfs
See also:
- https://www.linuxfromscratch.org/blfs/view/svn/postlfs/initramfs.html
- https://wiki.debian.org/initramfs
- https://en.wikipedia.org/wiki/Initramfs
- http://www.linuxfromscratch.org/blfs/view/svn/postlfs/initramfs.html
rescueInitramfs
cramfs, squashfs
(squashfs has replaced cramfs)
Read-only filesystem with low overhead, most useful for embedded devices, thin clients, system partitions on phones, liveCD/liveUSB.
In some cases the read-only nature is part of the design and means an embedded device can't easily brick itself, in other cases (e.g. liveUSB) it's mainly to save a little space.