Re: io performance...

[Max Waterman <davidmaxwaterman+kernel@fastmail.co.uk>]

Robert Hancock wrote:
> Jeff V. Merkey wrote:
> > Max Waterman wrote:
> >
> > > One further question. I get these messages 'in' dmesg :
> > >
> > > sda: asking for cache data failed
> > > sda: assuming drive cache: write through
> > >
> > > How can I force it to be 'write back'?
> >
> > Forcing write back is a very bad idea unless you have a battery backed 
> > up RAID controller.  
>
> This is not what these messages are referring to. Those write through 
> vs. write back messages are referring to detecting the drive write cache 
> mode, not setting it. Whether or not the write cache is enabled is used 
> to determine whether the sd driver uses SYNCHRONIZE CACHE commands to 
> flush the write cache on the device. If the drive says its write cache 
> is off or doesn't support determining the cache status, the kernel will 
> not issue SYNCHRONIZE CACHE commands. This may be a bad thing if the 
> device is really using write caching..
>     

So, if I have my raid controller set to use write-back, it *is* caching 
the writes, and so this *is* a bad thing, right?

If so, how to fix?

Max.
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