Disable read/write caching to disk?

[Bjoern Koenig]

Eric Anderson wrote:
> Is it possible to disable all read and write caching to a disk?

You can disable write the cache by adding the line

   hw.ata.wc="0"

to /boot/loader.conf. I'm not sure if there are methods to disable the 
read cache of the hard disk drive. I can't imagine that the read cache 
is relevant concerning file system consistency.

You can mount partitions that I/O will done synchronously by adding the 
option 'sync' to fstab.

> Also - what exactly are the issues with forcing an unclean filesystem to 
> mount rw?  I know you could (will?) have data corruption, but on which 
> data?  Only data that has not been synced to disk?  Any random amount of 
> data?

If you use soft updates, then you will notice that some files disappear. 
This is not corruption, but rather a loss. In theory you won't have 
corruption under certain circumstances, practically it's a controversial 
issue.

My machine crashes around three or four times a week for the last four 
month and I still don't have serious problems with the filesystem 
consistency.

Read more about soft updates in the handbook:
http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/configtuning-disk.html#SOFT-UPDATES

There are some discussions about this topic in the mailing list archive.

Regards
Björn