Well, of course, they are great if used consciously, I suppose.
In other words, this means they shouldn't be abused.
The key points about RAM disks are:
- They consume precious physical RAM.
According to ramdisk(7D) they can use up to 25% of RAM;
(as far as I suppose its pages aren't swappable)
(perhaps ipcs(1) and pmap(1) could help as in ISM run sample 3.0)
- They are ephemeral.
(they do not persist across reboots)
Naturally, at first no RAM disks exist in the system by default:
# ramdiskadm
# ls -al /dev/ramdisk
/dev/ramdisk: No such file or directory
# swap -sh
total: 1.9G allocated + 1.8G reserved = 3.7G used, 6.8G available
To create a RAM disk I must specify its name and size.
If you specify a size that cannot be honored and error message is printed out.
Otherwise, the new device name associated with the disk is printed out.
# ramdiskadm -a cache-01 512m
ramdiskadm: couldn't create ramdisk "cache-01":
Resource temporarily unavailable
# ramdiskadm -a cache-01 256m
/dev/ramdisk/cache-01
# ramdiskadm
Block Device Size Removable
/dev/ramdisk/cache-01 268435456 Yes
# swap -sh
total: 1.9G allocated + 1.8G reserved = 3.7G used, 6.5G available
The associated block file name is:
# ls -l /dev/ramdisk
total 1
... cache-01 -> ../../devices/pseudo/ramdisk@1024:cache-01
The associated raw file name is:
# ls -l /dev/rramdisk
total 1
... cache-01 -> ../../devices/pseudo/ramdisk@1024:cache-01,raw
In case the RAM disk must be disposed of before the next reboot:
(perhaps a RAM shortage is taking place and space must be freed)
# ramdiskadm -d cache-01
As an example, a RAM disk could perhaps be used as the raw device for a temporary ZFS pool intended for some very specialized scenario.
