Re: File type nomenclature in 170 Lassi Kortela 31 Jul 2020 16:36 UTC
> What you need to know in practice is which device driver a paticular > device node is using: disk, network, terminal, etc. And it varies by OS > whether e.g. a disk is a block or character device. On Linux the only > block devices left seem to be disks; on FreeBSD there are none. Here's the rundown from the FreeBSD handbook: <https://www.freebsd.org/doc/en/books/arch-handbook/driverbasics-block.html> > 9.4. Block Devices (Are Gone) > > Other UNIX® systems may support a second type of disk device known as block devices. Block devices are disk devices for which the kernel provides caching. This caching makes block-devices almost unusable, or at least dangerously unreliable. The caching will reorder the sequence of write operations, depriving the application of the ability to know the exact disk contents at any one instant in time. > > This makes predictable and reliable crash recovery of on-disk data structures (filesystems, databases, etc.) impossible. Since writes may be delayed, there is no way the kernel can report to the application which particular write operation encountered a write error, this further compounds the consistency problem. > > For this reason, no serious applications rely on block devices, and in fact, almost all applications which access disks directly take great pains to specify that character (or “raw”) devices should always be used. Because the implementation of the aliasing of each disk (partition) to two devices with different semantics significantly complicated the relevant kernel code FreeBSD dropped support for cached disk devices as part of the modernization of the disk I/O infrastructure. It sounds like block devices are only now used for disks, and the only reason you'd want to test for them is to avoid using them.