[Xen-devel] !!!!!help!I wouldn't be able to meet the deadline!(qcow format image file read operation in qemu-img-xen)[updated]

[hxkhust <hxkhust@xxxxxxx>]

Hi,guys,

During a HVM's running which take a qcow format image file as its own virtual disk, the qcow image file will be always read.In the situation that its qcow format image is based on a raw format image,  if nesethe backingfile ,just that raw format image file,would be read .my purpose is to cache the data that is read from the backingfile when the hvm is running .

Now what I concern is the following (which is in the /xen-4.1.2/tools/ioemu-qemu-xen/block-qcow.c) :

static void qcow_aio_read_cb(void *opaque, int ret)

{

........

if (!acb->cluster_offset) {

        if (bs->backing_hd) {

            /* read from the base image */

            acb->hd_aiocb = bdrv_aio_read(bs->backing_hd,                           //*************

                acb->sector_num, acb->buf, acb->n, qcow_aio_read_cb, acb);   //**************

//I read what the acb->buf points to, but find the reading operation is not finished. 

            if (acb->hd_aiocb == NULL)

                goto fail;

        } else {

            /* Note: in this case, no need to wait */

            memset(acb->buf, 0, 512 * acb->n);

            goto redo;

        }

    } else if (acb->cluster_offset & QCOW_OFLAG_COMPRESSED) {

        /* add AIO support for compressed blocks ? */

        if (decompress_cluster(s, acb->cluster_offset) < 0)

            goto fail;

        memcpy(acb->buf,

               s->cluster_cache + index_in_cluster * 512, 512 * acb->n);

        goto redo;

 .........

//********************************************************************************************8

when the statement:

acb->hd_aiocb = bdrv_aio_read(bs->backing_hd,

                acb->sector_num, acb->buf, acb->n, qcow_aio_read_cb, acb);n

has been completed, the content which the acb->buf points to has not been prepared.This is a asynchronous read operation.Who could tell me the principle or process about this  asynchronous read operation about these codes? if you describe it using the codes in xen,that will be so kind of you.I need to know when  the data has been copied to the memory which the acb->buf points to, and this problem is important to me.as the title mentioned ,I have to solve it as soon as possible.Or could you tell me how to cache the data which is read from the backingfile when a qcow image is regarded as a virtual disk in a running  HVM?

A newbie

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