Re: [Qemu-devel] [PATCH v5 3/4] qcow2: add shrink image support

[Kevin Wolf]

Am 13.07.2017 um 19:28 hat Pavel Butsykin geschrieben:
> On 13.07.2017 17:36, Max Reitz wrote:
> >On 2017-07-13 10:41, Kevin Wolf wrote:
> >>Am 12.07.2017 um 18:58 hat Max Reitz geschrieben:
> >>>On 2017-07-12 16:52, Kevin Wolf wrote:
> >>>>Am 12.07.2017 um 13:46 hat Pavel Butsykin geschrieben:
> >>>>>This patch add shrinking of the image file for qcow2. As a result, this 
> >>>>>allows
> >>>>>us to reduce the virtual image size and free up space on the disk without
> >>>>>copying the image. Image can be fragmented and shrink is done by 
> >>>>>punching holes
> >>>>>in the image file.
> >>>>>
> >>>>>Signed-off-by: Pavel Butsykin <address@hidden>
> >>>>>Reviewed-by: Max Reitz <address@hidden>
> >>>>>---
> >>>>>  block/qcow2-cluster.c  |  40 ++++++++++++++++++
> >>>>>  block/qcow2-refcount.c | 110 
> >>>>> +++++++++++++++++++++++++++++++++++++++++++++++++
> >>>>>  block/qcow2.c          |  43 +++++++++++++++----
> >>>>>  block/qcow2.h          |  14 +++++++
> >>>>>  qapi/block-core.json   |   3 +-
> >>>>>  5 files changed, 200 insertions(+), 10 deletions(-)
> >>>>>
> >>>>>diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> >>>>>index f06c08f64c..518429c64b 100644
> >>>>>--- a/block/qcow2-cluster.c
> >>>>>+++ b/block/qcow2-cluster.c
> >>>>>@@ -32,6 +32,46 @@
> >>>>>  #include "qemu/bswap.h"
> >>>>>  #include "trace.h"
> >>>>>+int qcow2_shrink_l1_table(BlockDriverState *bs, uint64_t exact_size)
> >>>>>+{
> >>>>>+    BDRVQcow2State *s = bs->opaque;
> >>>>>+    int new_l1_size, i, ret;
> >>>>>+
> >>>>>+    if (exact_size >= s->l1_size) {
> >>>>>+        return 0;
> >>>>>+    }
> >>>>>+
> >>>>>+    new_l1_size = exact_size;
> >>>>>+
> >>>>>+#ifdef DEBUG_ALLOC2
> >>>>>+    fprintf(stderr, "shrink l1_table from %d to %d\n", s->l1_size, 
> >>>>>new_l1_size);
> >>>>>+#endif
> >>>>>+
> >>>>>+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_WRITE_TABLE);
> >>>>>+    ret = bdrv_pwrite_zeroes(bs->file, s->l1_table_offset +
> >>>>>+                                       sizeof(uint64_t) * new_l1_size,
> >>>>>+                             (s->l1_size - new_l1_size) * 
> >>>>>sizeof(uint64_t), 0);
> >>>>>+    if (ret < 0) {
> >>>>>+        return ret;
> >>>>>+    }
> >>>>>+
> >>>>>+    ret = bdrv_flush(bs->file->bs);
> >>>>>+    if (ret < 0) {
> >>>>>+        return ret;
> >>>>>+    }
> >>>>
> >>>>If we have an error here (or after a partial bdrv_pwrite_zeroes()), we
> >>>>have entries zeroed out on disk, but in memory we still have the
> >>>>original L1 table.
> >>>>
> >>>>Should the in-memory L1 table be zeroed first? Then we can't
> >>>>accidentally reuse stale entries, but would have to allocate new ones,
> >>>>which would get on-disk state and in-memory state back in sync again.
> >>>
> >>>Yes, I thought of the same.  But this implies that the allocation is
> >>>able to modify the L1 table, and I find that unlikely if that
> >>>bdrv_flush() failed already...
> >>>
> >>>So I concluded not to have an opinion on which order is better.
> >>
> >>Well, let me ask the other way round: Is there any disadvantage in first
> >>zeroing the in-memory table and then writing to the image?
> >
> >I was informed that the code would be harder to write. :-)
> >
> >>If I have a choice between "always safe" and "not completely safe, but I
> >>think it's unlikely to happen", especially in image formats, then I will
> >>certainly take the "always safe".
> >>
> >>>>>+    BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_FREE_L2_CLUSTERS);
> >>>>>+    for (i = s->l1_size - 1; i > new_l1_size - 1; i--) {
> >>>>>+        if ((s->l1_table[i] & L1E_OFFSET_MASK) == 0) {
> >>>>>+            continue;
> >>>>>+        }
> >>>>>+        qcow2_free_clusters(bs, s->l1_table[i] & L1E_OFFSET_MASK,
> >>>>>+                            s->cluster_size, QCOW2_DISCARD_ALWAYS);
> >>>>>+        s->l1_table[i] = 0;
> >>>>>+    }
> >>>>>+    return 0;
> >>>>>+}
> >>>>>+
> >>>>>  int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
> >>>>>                          bool exact_size)
> >>>>>  {
> >>>>
> >>>>I haven't checked qcow2_shrink_reftable() for similar kinds of problems,
> >>>>I hope Max has.
> >>>
> >>>Well, it's exactly the same there.
> >>
> >>Ok, so I'll object to this code without really having looked at it.
> >I won't object to your objection. O:-)
> 
> Kevin,
> 
> Can you help me to reduce the number of patch-set versions? :)
> And look at the rest part of the series, thanks!

Patches 1 and 2 looked good to me. I didn't have a look at the test
case, but if it passes and Max reviewed it, it can't be too bad. ;-)

Kevin