How to disable hard disk write cache?

[Xin Li]

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On 9/16/14 5:36 PM, Xu Zhe wrote:
> Hi, all,
> 
> Does anyone knows how to disable write cache of hard disk?
> 
> I have found some hints here at Freebsd website:
> 
> https://www.freebsd.org/doc/handbook/configtuning-disk.html
> 
> However, this seems only work for ATA devices, what about SAS/NLSAS
> devices (Meanwhile, it seems that there is no such sysctl in latest
> Freebsd release, which is 10.0)?
> 
> Any hints are welcomed!

Why do you want to disable write cache in the first place?  It's not
needed for most configurations nowadays.

Modern SATA/SAS/SCSI devices usually comes with the capability of
tagged commands, allowing the OS to know when a write buffer is on
stable storage.  With this, file systems can easily implement the
right semantics and recover from e.g. a power outage, etc.

Cheers,
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