libblkid library is used to identify block devices (disks) as to their content (e.g.
filesystem type) as well as extracting additional information such as
filesystem labels/volume names, unique identifiers/serial numbers, etc.
A common use is to allow use of LABEL= and UUID= tags instead of hard-coding
specific block device names into configuration files.
Block device information is normally kept in a cache file
/etc/blkid.tab and is verified to still be valid before being returned to the user
(if the user has read permission on the raw block device, otherwise not).
The cache file also allows unprivileged users (normally anyone other
than root, or those not in the "disk" group) to locate devices by label/id.
The standard location of the cache file can be overridden by the
environment variable BLKID_FILE.
In situations where one is getting information about a single known device,
it does not impact performance whether the cache is used or not (unless you
are not able to read the block device directly). If you are dealing with
multiple devices, use of the cache is highly recommended (even if empty) as
devices will be scanned at most one time and the on-disk cache will be
updated if possible. There is rarely a reason not to use the cache.
In some cases (modular kernels), block devices are not even visible until
after they are accessed the first time, so it is critical that there is
some way to locate these devices without enumerating only visible devices,
so the use of the cache file is
required in this situation.