Re: [Qemu-devel] 'qemu-nbd' explicite flush to disk

[Mark Trumpold]

Paolo,

Please recall you had sent me the link below to a kernel patch to allow
'qemu-nbd' to flush to disk.  The patch was applied with good success.

However, recent testing indicates that filesystem corruption issue I am
seeing seems to be tied to large (>500mbyte) filesystem writes.  I also
observe the kernel 'flush' thread approaching 100% cpu at times, and i/o
wait % as reported by 'top' increasing sharply during the copy/write.

Was wondering if perhaps there are any special kernel config options that
I may be missing for 'nbd', or any other information that might help.

Best regards,
Mark Trumpold


>-----Original Message-----
>From: Paolo Bonzini [mailto:address@hidden On Behalf Of Paolo
>Bonzini
>Sent: Wednesday, September 12, 2012 11:30 PM
>To: Mark Trumpold
>Cc: address@hidden
>Subject: Re: 'qemu-nbd' explicite flush to disk
>Il 12/09/2012 23:28, Mark Trumpold ha scritto:
>> So, I've been experimenting with 'qemu-nbd --cache=writeback ..'
>> This nicely eliminates the 'checkpoint' issue; however, I have as
>> yet been unable to explicitely flush things to disk -- which I would
>>like to
>> do just before a 'nilfs' snapshot.

>The Linux kernel driver for NBD does not support flushes.  Patches were
>sent to the maintainer, but he never applied them.
>You can get them at
>http://thread.gmane.org/gmane.linux.drivers.nbd.general/1108 and try
>them.
>> Subsequently I've been trying to call 'bdrv_co_flush(bs)' directly, but
>>I can't
>figure out how to dereference 'bs' for the call.
>>
>> I'm probably out in the weeds on this one.
>> Any guidance would be greatly appreciated.
>>
>> I am running:
>>     qemu-1.2.0
>>     linux kernel 3.3.1
>>
>> Thank you,
>> Mark Trumpold