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SubjectRe: Implementing NVMHCI...
Avi Kivity wrote:
> 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.

Makes a lot of sense...

>> 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.

Sure. "in the driver" is a highly relative phrase :) If there is code
to be shared among multiple callsites, let's share it.

>> (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.

At the kernel level, you mean?

Filesystems already must deal with issues such as avoiding RAID stripe
boundaries (man mke2fs, search for 'RAID').

So I hope that same code should be applicable to cases where the
"logical sector size" (as exported by storage interface) differs from
"physical sector size" (the underlying hardware sector size, not
directly accessible by OS).

But if you are talking about filesystems directly supporting sector
sizes >4kb, well, I'll let Linus and others settle that debate :) I
will just write the driver once the dust settles...


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