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Re: [Qemu-devel] [PATCH v2 01/11] block: Add op blocker type "device IO"

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v2 01/11] block: Add op blocker type "device IO"
Date: Wed, 13 May 2015 17:09:52 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 13/05/2015 17:02, Fam Zheng wrote:
>> > For example, SCSI requests can result in many consecutive I/Os:
>> > 
>> > (1) FUA requests are split in write+flush
>> > 
>> > (2) adapters that do not use QEMUSGList-based I/O only read 128K at a time
>> > 
>> > (3) WRITE SAME operations are also split in chunks
>> > 
>> > (4) UNMAP operations process one descriptor at a time
> I don't understand the point of these examples. If we don't return -EBUSY 
> here,
> the request will sneak into block/io.c and perhaps break qmp transaction
> semantics, if it lands between two backups.

It won't, because after blocking DEVICE_IO you will always drain I/O and
the bdrv_drain will loop until the above are all satisfied.


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