4K sectors

4K sectors

[James Harper]

I notice on my system that bcache presents 4K sectors to Linux. This is causing me problems with Windows on Xen using my PV drivers (problem = doesn't work). I haven't figured out if it's my PV drivers that don't tell Windows the right thing, or if Windows just doesn't like 4K sectors.

Is the 4K sector thing intentional? I assume there is a performance improvement in doing so.

And what would be the performance impact if I just told Windows that it was dealing with 512 byte sectors (eg fake it)?

Thanks

James

Re: 4K sectors

[Kent Overstreet]

On Mon, Aug 13, 2012 at 07:19:50AM +0000, James Harper wrote:
> I notice on my system that bcache presents 4K sectors to Linux. This is causing me problems with Windows on Xen using my PV drivers (problem = doesn't work). I haven't figured out if it's my PV drivers that don't tell Windows the right thing, or if Windows just doesn't like 4K sectors.
> 
> Is the 4K sector thing intentional? I assume there is a performance improvement in doing so.

It is intentional, but it's configurable. It's mainly for devices that
don't support < 4k writes; there might also be a performance impact with
some SSDs but I'm sure it's slight.

You'll need to reformat - pass --block 512 to make-bcache.  The backing
devices need to be reformatted too, but just rerunning make-bcache won't
touch any of the existing data.

> 
> And what would be the performance impact if I just told Windows that it was dealing with 512 byte sectors (eg fake it)?
> 
> Thanks
> 
> James
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