Re: Implementing NVMHCI...

[Avi Kivity <avi@redhat.com>]

Jeff Garzik wrote:
> Speaking of RMW...    in one sense, we have to deal with RMW anyway. 
> Upcoming ATA hard drives will be configured with a normal 512b sector 
> API interface, but underlying physical sector size is 1k or 4k.
>
> The disk performs the RMW for us, but we must be aware of physical 
> sector size in order to determine proper alignment of on-disk data, to 
> minimize RMW cycles.
>

Virtualization has the same issue.  OS installers will typically setup 
the first partition at sector 63, and that means every page-sized block 
access will be misaligned.  Particularly bad when the guest's disk is 
backed on a regular file.

Windows 2008 aligns partitions on a 1MB boundary, IIRC.

> At the moment, it seems like most of the effort to get these ATA 
> devices to perform efficiently is in getting partition / RAID stripe 
> offsets set up properly.
>
> So perhaps for NVMHCI we could
> (a) hardcode NVM sector size maximum at 4k
> (b) do RMW in the driver for sector size >4k, and

Why not do it in the block layer?  That way it isn't limited to one driver.

> (c) export information indicating the true sector size, in a manner 
> similar to how the ATA driver passes that info to userland 
> partitioning tools.

Eventually we'll want to allow filesystems to make use of the native 
sector size.

-- 
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