Re: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Alasdair G Kergon]

On Mon, May 28, 2007 at 11:30:32AM +1000, Neil Brown wrote:
> 1/ A BIO_RW_BARRIER request should never fail with -EOPNOTSUP.
 
The device-mapper position has always been that we require

>  a zero-length BIO_RW_BARRIER 

(i.e. containing no data to read or write - or emulated, possibly
device-specific)

before we can provide full barrier support.
  (Consider multiple active paths - each must see barrier.)

Until every device supports barriers -EOPNOTSUP support is required.
  (Consider reconfiguration of stacks of devices - barrier support is a
   dynamic block device property that can switch between available and
   unavailable at any time.)

Alasdair
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