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Disabling disk caches [Re: CVS commit: src/lib/libc/sys, re sync(1)]

On Wed, Mar 25, 2009 at 09:07:43AM +0000, David Holland wrote:

>  > +In practice there are many possible ways for this mechanism to go
>  > +astray.
>  > +It is prudent (where possible) to allow a few seconds after syncing
>  > +for everything to settle before e.g. turning off the power.
>  > +.Pp
>  > +It may also be desirable to use
>  > +.Xr dkctl 8
>  > +or
>  > +.Xr scsictl 8
>  > +to disable the write-back cache entirely.
>  > 
>  > This is wishy washy. It gives the sense that we know there is a problem, 
> but
>  > simply couldn't be arsed fixing it. What do you want to convey?
> Should we have a list of filesystems and whether they work properly?
> Crossreferences to mount_foo(8) for all foo? Or should it just contain
> Kirk's advice, that is, people who care about their data shouhld
> always disable the write-back cache?

I think we should:

(a) Automatically disable the disk write cache the moment a file system is
    mounted on the disk, unless that file system is known to handle caching.
    For our purposes, that leaves logging ffs and zfs (and whatever other
    modern file systems come along) with the cache enabled.

(b) Change mount to flush the disk cache as a test, if r/w mount. If we get
    an error, send a warning message to the log. This will help us to get
    broken disk drivers fixed.


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