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

[Max Reitz]

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:-)

Max

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