Re: [Qemu-devel] Regression: block: Add .bdrv_co_pwrite_zeroes()

[Eric Blake <eblake@redhat.com>]

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On 07/04/2016 07:49 AM, Peter Lieven wrote:
> Hi,
> 
> the above commit:
> 
> commit d05aa8bb4a8b6aa9a915ec5074fb12ae632d2323
> Author: Eric Blake <eblake@redhat.com>
> Date:   Wed Jun 1 15:10:03 2016 -0600
> 
>     block: Add .bdrv_co_pwrite_zeroes()
> 
> introduces a regression (at least for me).
> 
> The Limits from the iSCSI Block Limits VPD have no requirement of being
> a power of two.
> We use Dell Equallogic iSCSI SANs for instance. They have an internal
> page size of 15MB. And
> they advertise this page size as max_ws_len, opt_transfer_len and
> opt_discard_alignment.

A non-power-of-2 max_ws_len shouldn't be a problem, but opt_transfer_len
and opt_discard_alignment not being a power of 2 impacts other code.
15MB is a rather odd page size.

> 
> I think we cannot assert that that these alignments are a power of 2.

Perhaps that means we should just fix our code to round things down to
the nearest power of 2 (8MB) for the opt_transfer_len and
opt_discard_alignment values.  Can you post a stack-trace of the actual
assertion you are hitting?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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