Disable read/write caching to disk?
Eric Anderson
anderson at centtech.com
Thu May 26 10:40:42 PDT 2005
Bjoern Koenig wrote:
> 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"
That appears to be for ata devices only, correct? I'm using fiber
channel so it's scsi-like.
> 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.
Host A has filesystem mounted rw, host B has it mounted ro. If host A
makes changes, host B cannot see them without unmounting and remounting
ro. I wanted to find out if it was a read caching issue on host B.
> You can mount partitions that I/O will done synchronously by adding the
> option 'sync' to fstab.
I've tried this. I'm not sure if it made any difference.
>> 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.
I've turned off softupdates to minimize my variables.
> 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.
I'll look again.. Thanks
Eric
--
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Eric Anderson Sr. Systems Administrator Centaur Technology
A lost ounce of gold may be found, a lost moment of time never.
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