On Thu, Mar 07, 2013 at 04:59:05PM +0100, Stefan Hajnoczi wrote:
> On Wed, Mar 06, 2013 at 09:48:43AM -0500, Jeff Cody wrote:
> > @@ -958,12 +960,150 @@ exit:
> > return ret;
> > }
> >
> > +/*
> > + * Allocate a new payload block at the end of the file.
> > + *
> > + * Allocation will happen at 1MB alignment inside the file
> > + *
> > + * Returns the file offset start of the new payload block
> > + */
> > +static int vhdx_allocate_block(BlockDriverState *bs, BDRVVHDXState *s,
> > + uint64_t *new_offset)
> > +{
> > + *new_offset = bdrv_getlength(bs->file);
> > +
> > + /* per the spec, the address for a block is in units of 1MB */
> > + if (*new_offset % (1024*1024)) {
> > + *new_offset = ((*new_offset >> 20) + 1) << 20; /* round up to 1MB
> > */
> > + }
> > + return bdrv_truncate(bs->file, *new_offset + s->block_size);
> > +}
>
> Is it necessary to resize the file? Why not just write at EOF to grow
> it?
>
After the recent bdrv_truncate() discussion, I think that may be best.
> > +/*
> > + * Update the BAT tablet entry with the new file offset, and the new entry
> > + * state */
> > +static int vhdx_update_bat_table_entry(BlockDriverState *bs, BDRVVHDXState
> > *s,
> > + vhdx_sector_info *sinfo, int state)
> > +{
> > + uint64_t bat_tmp;
> > + uint64_t bat_entry_offset;
> > +
> > + /* The BAT entry is a uint64, with 44 bits for the file offset in
> > units of
> > + * 1MB, and 3 bits for the block state. */
> > + s->bat[sinfo->bat_idx] = ((sinfo->file_offset>>20) <<
> > + VHDX_BAT_FILE_OFF_BITS);
> > +
> > + s->bat[sinfo->bat_idx] |= state & VHDX_BAT_STATE_BIT_MASK;
> >
> > + bat_tmp = cpu_to_le64(s->bat[sinfo->bat_idx]);
> > + bat_entry_offset = s->bat_offset + sinfo->bat_idx *
> > sizeof(vhdx_bat_entry);
> > +
> > + return bdrv_pwrite_sync(bs->file, bat_entry_offset, &bat_tmp,
> > + sizeof(vhdx_bat_entry));
> > +}
> >
> > static coroutine_fn int vhdx_co_writev(BlockDriverState *bs, int64_t
> > sector_num,
> > int nb_sectors, QEMUIOVector *qiov)
> > {
> > - return -ENOTSUP;
> > + int ret = -ENOTSUP;
> > + BDRVVHDXState *s = bs->opaque;
> > + vhdx_sector_info sinfo;
> > + uint64_t bytes_done = 0;
> > + QEMUIOVector hd_qiov;
> > +
> > + qemu_iovec_init(&hd_qiov, qiov->niov);
> > +
> > + qemu_co_mutex_lock(&s->lock);
> > +
> > + /* Per the spec, on the first write of guest-visible data to the file
> > the
> > + * data write guid must be updated in the header */
> > + if (s->first_visible_write) {
> > + s->first_visible_write = false;
> > + vhdx_update_headers(bs, s, true);
> > + }
> > +
> > + while (nb_sectors > 0) {
> > + if (s->params.data_bits & VHDX_PARAMS_HAS_PARENT) {
> > + /* not supported yet */
> > + ret = -ENOTSUP;
> > + goto exit;
> > + } else {
> > + vhdx_block_translate(s, sector_num, nb_sectors, &sinfo);
> > +
> > + qemu_iovec_reset(&hd_qiov);
> > + qemu_iovec_concat(&hd_qiov, qiov, bytes_done,
> > sinfo.bytes_avail);
> > + /* check the payload block state */
> > + switch (s->bat[sinfo.bat_idx] & VHDX_BAT_STATE_BIT_MASK) {
> > + case PAYLOAD_BLOCK_ZERO:
> > + /* in this case, we need to preserve zero writes for
> > + * data that is not part of this write, so we must pad
> > + * the rest of the buffer to zeroes */
> > +
> > + /* if we are on a posix system with ftruncate() that
> > extends
> > + * a file, then it is zero-filled for us. On Win32, the
> > raw
> > + * layer uses SetFilePointer and SetFileEnd, which does not
> > + * zero fill AFAIK */
> > +
> > + /* TODO: queue another write of zero buffers if the host
> > OS does
> > + * not zero-fill on file extension */
> > +
> > + /* fall through */
> > + case PAYLOAD_BLOCK_NOT_PRESENT: /* fall through */
> > + case PAYLOAD_BLOCK_UNMAPPED: /* fall through */
> > + case PAYLOAD_BLOCK_UNDEFINED: /* fall through */
> > + ret = vhdx_allocate_block(bs, s, &sinfo.file_offset);
> > + if (ret < 0) {
> > + goto exit;
> > + }
> > + /* once we support differencing files, this may also be
> > + * partially present */
> > + /* update block state to the newly specified state */
> > + ret = vhdx_update_bat_table_entry(bs, s, &sinfo,
> > +
> > PAYLOAD_BLOCK_FULL_PRESENT);
> > + if (ret < 0) {
> > + goto exit;
> > + }
>
> The BAT table entry must be written after data is already on disk.
> Otherwise a crash partway through would leave the BAT table pointing to
> undefined data.
>
> Do you need to use the log to ensure this?
Yes, indeed. The log support is what I am working on now - all
metadata and BAT writes are supposed to go through the log, for this
reason. The next version will have log support, and so I will use
that for the BAT writes (and any other metadata writes, except the
header).
