[Qemu-devel] guest sync and bdrv_co_flush

[Qemu-devel] guest sync and bdrv_co_flush

[lihuiba]

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Hi, all


I'm a user of qemu/kvm, and I'm wondering some internals of qemu/kvm, so I'd better post it in 
this developer's mailing list.


To be specific, I'm wondering how data is flushed to disk. Intuitively, when the guest issues a 
SYNCHRONIZE CACHE command in the SCSI layer, qemu/kvm should call bdrv_co_flush(),
which will eventually call the block driver's bdrv_co_flush_to_os() and bdrv_co_flush_to_disk().


But from simple grep-ing, I didn't find any calling to bdrv_co_flush() is responsible for SYNCHRONIZE 
CACHE. So, can you tell me how qemu/kvm ensures guest data be written on persistent storage?


Thank you, very much!

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Re: [Qemu-devel] guest sync and bdrv_co_flush

[Kevin Wolf]

Am 07.11.2014 um 09:39 hat lihuiba geschrieben:
> Hi, all
> 
> I'm a user of qemu/kvm, and I'm wondering some internals of qemu/kvm, so I'd
> better post it in 
> this developer's mailing list.
> 
> To be specific, I'm wondering how data is flushed to disk. Intuitively, when
> the guest issues a 
> SYNCHRONIZE CACHE command in the SCSI layer, qemu/kvm should call bdrv_co_flush
> (),
> which will eventually call the block driver's bdrv_co_flush_to_os()
> and bdrv_co_flush_to_disk().
> 
> But from simple grep-ing, I didn't find any calling to bdrv_co_flush() is
> responsible for SYNCHRONIZE 
> CACHE. So, can you tell me how qemu/kvm ensures guest data be written on
> persistent storage?

The relevant code is in hw/scsi/scsi-disk.c, in the function
scsi_disk_emulate_command(), for 'case SYNCHRONIZE_CACHE:'.

The call chain from there is blk_aio_flush() -> bdrv_aio_flush() ->
bdrv_aio_flush_co_entry() -> bdrv_co_flush()

Kevin