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

[Christian Borntraeger]

On 23/01/14 11:29, Kevin Wolf wrote:
> 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.

Yes, thats what we do now, but it requires a command line parameter. 
A year ago or so we posted patches for block size detection (as well as geometry
pass through) but the necessary rework after feedback is still pending
(this got lost in a bunch of other todos, but we will restart that)

> 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?

Yes, its mostly the boot process. But there is also a problem if you have
an image file on file system that is on a 4k disk. Then the guest might send
non-aligned request IIRC.
> 
>> 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
>