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



On Thu, 31 May 2007, Jens Axboe wrote:

On Thu, May 31 2007, Phillip Susi wrote:
David Chinner wrote:
That sounds like a good idea - we can leave the existing
WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
behaviour that only guarantees ordering. The filesystem can then
choose which to use where appropriate....

So what if you want a synchronous write, but DON'T care about the order?
  They need to be two completely different flags which you can choose
to combine, or use individually.

If you have a use case for that, we can easily support it as well...
Depending on the drive capabilities (FUA support or not), it may be
nearly as slow as a "real" barrier write.

true, but a "real" barrier write could have significant side effects on other writes that wouldn't happen with a synchronous wrote (a sync wrote can have other, unrelated writes re-ordered around it, a barrier write can't)

David Lang


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