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[Qemu-devel] Re: [PATCH] Disable O_DIRECT for physical CDROM/DVD drives


From: Jes Sorensen
Subject: [Qemu-devel] Re: [PATCH] Disable O_DIRECT for physical CDROM/DVD drives
Date: Tue, 20 Jul 2010 18:02:04 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Lightning/1.0b2pre Thunderbird/3.0.5

On 07/20/10 17:40, Anthony Liguori wrote:
> On 07/20/2010 10:17 AM, address@hidden wrote:
>> From: Jes Sorensen<address@hidden>
>>
>> O_DIRECT (cache=none) requires sector alignment, however the physical
>> sector size of CDROM/DVD drives is 2048, as opposed to most disk
>> devices which use 512. QEMU is hard coding 512 all over the place, so
>> allowing O_DIRECT for CDROM/DVD devices does not work.
>>
>> Signed-off-by: Jes Sorensen<address@hidden>
>>    
> 
> Wouldn't a better solution be to have a cdrom_read/cdrom_write hook that
> did the appropriate bouncing?
> 
> Silently disabling something a user explicitly asked for is not a good
> option.  In the very least, it should error out entirely.

I thought about this, but it would require basically fixing up or
copying all of the pread/pwrite code to use the right block size. This
is really more of a band-aid but it should be pretty safe.

Cheers,
Jes



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