Re: xbd 32k transfer limit

[Christoph Egger <Christoph_Egger%gmx.de@localhost>]

On 03.05.11 20:11, Toby Karyadi wrote:
> On 5/3/2011, "Manuel Bouyer" <bouyer%antioche.eu.org@localhost> wrote:
> 
>>
>> The real limit is inside the include/xen3-public/io/blkif.h file, which
>> defines the interface between front and backend block devices.
>> The size of a transfer is limited by BLKIF_MAX_SEGMENTS_PER_REQUEST, which
>> is 11. With 4k pages, this makes a max transfer size of 44k. As we want
>> a power of 2, we use 32k.
>>
>>
> Well, no quick way around it, obviously. I bumped MAXPHYS to 64k and got
> an assert in xbd_xenbus.c during bootup of the domu, so that's cool. I
> checked xen 4.1 and it hasn't changed either since the reason for the
> define is to keep the struct in a memory page boundary (4k). 
> 
> So, I guess the lesson is that for xen backing store on a 3 disk raid5
> setup, stick to 32 sector stripe unit and 32k ffs block size. That is,
> unless somebody actually implements scatter gather mechanism (wink,
> wink, nudge, nudge).

vscsi PV drivers may be an alternative.

Christoph