Re: [Qemu-devel] [6397] Vectored block device API (Avi Kivity)

[Anthony Liguori]

Gerd Hoffmann wrote:
> Anthony Liguori wrote:
>   
> > For block devices, this translates to vectored I/O.  This patch implements
> > an aynchronous vectored interface for the qemu block devices.  At the moment
> > all I/O is bounced and submitted through the non-vectored API; in the future
> > we will convert block devices to natively support vectored I/O wherever
> > possible.
> >     
>
> Any plan for this?
>   

Avi is on vacation this week but my understanding is that he'll be 
submitting patches shortly after he gets back.

> Current state is this:  BlockDriver provides *three* ways to do I/O.
>
> #1 is brdv_{read,write}, operating on sectors.
> #2 is bdrv_aio_{read,write}, operation on sectors too.
> #3 is brdv_{pread,pwrite}, operating on bytes.
>
> All block drivers implement #1.
> Most block drivers implement only #1.
> #2 is implemented by qcow, qcow2, raw (including host_device).
> #3 is implemented by raw (+hostdevice) only.
>
> We can't kill #1 for the time being.
>
> Not sure what the motivation for #3 is (O_DIRECT ?).

Internally, qcow, qcow2, etc. use the block API to access the disk 
formats.  These accesses are not always sector aligned.

> For qcow2 this is a bit more difficuilt.  Metadata updating obviously
> requires some locking.  Probably lookups too (I'm not familiar with the
> on-disk format and driver internals).  The actual data transfer should
> be doable unlocked I think.
>
> The interface could look like this:
>
>     int (*bdrv_read)(BlockDriverState *bs, QEMUIOVector *qiov,
>                      int64_t sector_num);
>     int (*bdrv_write)(BlockDriverState *bs, QEMUIOVector *qiov,
>                       int64_t sector_num);
>
> All the aio magic can live in the block layer then, well hidden from the
> block drivers.
>   

I agree with this on principle, but making all of the entry points 
thread safe is a fair bit of work.  Recall that we don't use threads on 
every platform so we need to take that into account too.

Regards,

Anthony Liguori

> Comments on this?  Especially from people knowing qcow2 better that I do?
>
> cheers,
>   Gerd
>
>
>