Re: [Xen-devel] bug when using 4K sectors?

[Joseph Glanville <joseph.glanville@xxxxxxxxxxxxxx>]

On 6 September 2012 20:58, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> On Wed, Sep 05, 2012 at 11:56:08PM +0000, James Harper wrote:
>> > On Mon, Aug 13, 2012 at 02:12:58PM +0000, James Harper wrote:
>> > > I notice this code in drivers/block/xen-blkback/common.h
>> > >
>> > > #define vbd_sz(_v)      ((_v)->bdev->bd_part ? \
>> > >                          (_v)->bdev->bd_part->nr_sects : \
>> > >                           get_capacity((_v)->bdev->bd_disk))
>> > >
>> > > is the value returned by vbd_sz(_v) the number of sectors in the Linux
>> > > device (eg size / 4096), or the number of 512 byte sectors? I suspect
>> > > the former which is causing block requests beyond 1/8th the size of
>> > > the device to fail (assuming 4K sectors are expected to work at all -
>> > > I can't quite get my head around how it would be expected to work -
>> > > does Linux do the read-modify-write if required?)
>> >
>> > I think you need to instrument it to be sure.. But more interesting, do you
>> > actually have a disk that exposes a 4KB hardware and logical sector? So far
>> > I've only found SSDs that expose a 512kB logical sector but also expose the
>> > 4KB hardware.
>> >
>> > Never could figure out how that is all suppose to work as the blkback is 
>> > filled
>> > with << 9 on a bunch of things.
>> >
>>
>> I was using bcache which does expose a 4K block size, by default. I changed 
>> it to 512 and it all works now, although I haven't tested if there is any 
>> loss of performance.
>
> OK, let me see how I can setup bcache and play with that.
>>
>> Does Xen provide a way to tell Windows that the underlying device is 512e 
>> (4K sector with 512 byte emulated interface)? This would keep everything 
>> working as is but allow windows to align writes to 4K boundaries where 
>> possible.
>
> We can certainly expose that via the XenBus interface.
>>
>> James
>
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After reading through blkback it appears that it can only support 512
byte sector sizes and removing this limitation would take quite abit
of work.
It uses hard coded bitshifts pervasively to convert between number of
requests/pages and size of sectors etc. (that is all the >> 9
everywhere)

I am going to see what I can about working on getting it to support 4k
sectors too and eventually uncoupled logical/physical sizes but that
would take even more work as far as I can tell.

Being able to use 4k sectors seems like it would provide pretty
massive gains in performance just by being more efficient let alone
increasing byte aligned writes to the underlying block storage system.

Joseph.

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