On Wed, Apr 22, 2015 at 04:16:38PM +0300, Denis V. Lunev wrote:
> On 22/04/15 16:08, Stefan Hajnoczi wrote:
> >On Wed, Mar 11, 2015 at 01:28:02PM +0300, Denis V. Lunev wrote:
> >>+static int64_t allocate_cluster(BlockDriverState *bs, int64_t sector_num)
> >>+{
> >>+ BDRVParallelsState *s = bs->opaque;
> >>+ uint32_t idx, offset, tmp;
> >>+ int64_t pos;
> >>+ int ret;
> >>+
> >>+ idx = sector_num / s->tracks;
> >>+ offset = sector_num % s->tracks;
> >>+
> >>+ if (idx >= s->catalog_size) {
> >>+ return -EINVAL;
> >>+ }
> >>+ if (s->catalog_bitmap[idx] != 0) {
> >>+ return (uint64_t)s->catalog_bitmap[idx] * s->off_multiplier +
> >>offset;
> >>+ }
> >>+
> >>+ pos = bdrv_getlength(bs->file) >> BDRV_SECTOR_BITS;
> >>+ bdrv_truncate(bs->file, (pos + s->tracks) << BDRV_SECTOR_BITS);
> >>+ s->catalog_bitmap[idx] = pos / s->off_multiplier;
> >>+
> >>+ tmp = cpu_to_le32(s->catalog_bitmap[idx]);
> >>+
> >>+ ret = bdrv_pwrite_sync(bs->file,
> >>+ sizeof(ParallelsHeader) + idx * sizeof(tmp), &tmp,
> >>sizeof(tmp));
> >What is the purpose of the sync?
> This is necessary to preserve image consistency on crash from
> my point of view. There is no check consistency at the moment.
> The sync will be removed later when proper crash detection
> code will be added (patches 19, 20, 21)Let's look at possible orderings in case of failure: A. BAT update B. Data write This sync enforces A, B ordering. If we can see B, then A must also have happened thanks to the sync. But A, B ordering is too conservative. Imagine B, A ordering and the failure where we crash before A. It means we wrote the data but never linked it into the BAT. What happens in that case? We've leaked a cluster in the underlying image file but it doesn't corrupt the visible disk from the guest point-of-view. Because your implementation uses truncate to extend the file size before A, even the A, B failure case results in a leaked cluster. So the B, A case is not worse in any way. Why do other image formats sync cluster allocation updates? Because they support backing files and in that case an A, B ordering results in data corruption so they enforce B, A ordering (the opposite of what you're trying to do!). The reason why A, B ordering results in data corruption when backing files are in use is because the guest's write request might touch only a subset of the cluster (a couple of sectors out of the whole cluster). So the guest needs to copy the remaining sectors from the backing file. If there is a dangling BAT entry like in the A, B failure case, then the guest will see a zeroed cluster instead of the contents of the backing file. This is a data corruption, but only if a backing file is being used! So the sync is not necessary, both A, B and B, A ordering work for block/parallels.c. Stefan
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