Re: [Qemu-devel] [PATCH v3 00/29] block: Support for 512b-on-4k emulation

[Kevin Wolf]

Am 22.01.2014 um 21:30 hat Christian Borntraeger geschrieben:
> On 17/01/14 15:14, Kevin Wolf wrote:
> > This patch series adds code to the block layer that allows performing
> > I/O requests in smaller granularities than required by the host backend
> > (most importantly, O_DIRECT restrictions). It achieves this for reads
> > by rounding the request to host-side block boundary, and for writes by
> > performing a read-modify-write cycle (and serialising requests
> > touching the same block so that the RMW doesn't write back stale data).
> 
> Nice, this might really help on s390 (also for KVM) since dasd disks usually
> have a 4k sector size. We also have flash systems with 4k block size. Both
> disk systems cause lots of trouble with cache=none and friends.

Can't you configure guests to use 4k sector size as well? With these
patches, it should work, but there is an obvious performance penalty, so
you want the guest to make as little use of it as possible.

For PCs, it's essentially just the boot process that needs 512 byte
accesses because that's the BIOS interface, and the OS generally won't
send misaligned requests later.

What's the situation on s390?

> Do you have a tree with these patches, so that I can test those on s390?

Sure, and any testing is appreciated:

    git://repo.or.cz/qemu/kevin.git align

Kevin