2011/1/21 Kevin Wolf <kw...@redhat.com>:
> Am 21.01.2011 09:08, schrieb Pierre Riteau:
>> Le 20 janv. 2011 à 17:18, Yoshiaki Tamura <tamura.yoshi...@lab.ntt.co.jp> a
>> écrit :
>>
>>> 2011/1/20 Pierre Riteau <pierre.rit...@irisa.fr>:
>>>> On 20 janv. 2011, at 03:06, Yoshiaki Tamura wrote:
>>>>
>>>>> 2011/1/19 Pierre Riteau <pierre.rit...@irisa.fr>:
>>>>>> b02bea3a85cc939f09aa674a3f1e4f36d418c007 added a check on the return
>>>>>> value of bdrv_write and aborts migration when it fails. However, if the
>>>>>> size of the block device to migrate is not a multiple of BLOCK_SIZE
>>>>>> (currently 1 MB), the last bdrv_write will fail with -EIO.
>>>>>>
>>>>>> Fixed by calling bdrv_write with the correct size of the last block.
>>>>>> ---
>>>>>> block-migration.c | 16 +++++++++++++++-
>>>>>> 1 files changed, 15 insertions(+), 1 deletions(-)
>>>>>>
>>>>>> diff --git a/block-migration.c b/block-migration.c
>>>>>> index 1475325..eeb9c62 100644
>>>>>> --- a/block-migration.c
>>>>>> +++ b/block-migration.c
>>>>>> @@ -635,6 +635,8 @@ static int block_load(QEMUFile *f, void *opaque, int
>>>>>> version_id)
>>>>>> int64_t addr;
>>>>>> BlockDriverState *bs;
>>>>>> uint8_t *buf;
>>>>>> + int64_t total_sectors;
>>>>>> + int nr_sectors;
>>>>>>
>>>>>> do {
>>>>>> addr = qemu_get_be64(f);
>>>>>> @@ -656,10 +658,22 @@ static int block_load(QEMUFile *f, void *opaque,
>>>>>> int version_id)
>>>>>> return -EINVAL;
>>>>>> }
>>>>>>
>>>>>> + total_sectors = bdrv_getlength(bs) >> BDRV_SECTOR_BITS;
>>>>>> + if (total_sectors <= 0) {
>>>>>> + fprintf(stderr, "Error getting length of block device
>>>>>> %s\n", device_name);
>>>>>> + return -EINVAL;
>>>>>> + }
>>>>>> +
>>>>>> + if (total_sectors - addr < BDRV_SECTORS_PER_DIRTY_CHUNK) {
>>>>>> + nr_sectors = total_sectors - addr;
>>>>>> + } else {
>>>>>> + nr_sectors = BDRV_SECTORS_PER_DIRTY_CHUNK;
>>>>>> + }
>>>>>> +
>>>>>> buf = qemu_malloc(BLOCK_SIZE);
>>>>>>
>>>>>> qemu_get_buffer(f, buf, BLOCK_SIZE);
>>>>>> - ret = bdrv_write(bs, addr, buf,
>>>>>> BDRV_SECTORS_PER_DIRTY_CHUNK);
>>>>>> + ret = bdrv_write(bs, addr, buf, nr_sectors);
>>>>>>
>>>>>> qemu_free(buf);
>>>>>> if (ret < 0) {
>>>>>> --
>>>>>> 1.7.3.5
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>> Hi Pierre,
>>>>>
>>>>> I don't think the fix above is correct. If you have a file which
>>>>> isn't aliened with BLOCK_SIZE, you won't get an error with the
>>>>> patch. However, the receiver doesn't know how much sectors which
>>>>> the sender wants to be written, so the guest may fail after
>>>>> migration because some data may not be written. IIUC, although
>>>>> changing bytestream should be prevented as much as possible, we
>>>>> should save/load total_sectors to check appropriate file is
>>>>> allocated on the receiver side.
>>>>
>>>> Isn't the guest supposed to be started using a file with the correct size?
>>>
>>> I personally don't like that; It's insisting too much to the user.
>>> Can't we expand the image on the fly? We can just abort if expanding
>>> failed anyway.
>>
>> At first I thought your expansion idea was best, but now I think there are
>> valid scenarios where it fails.
>>
>> Imagine both sides are not using a file but a disk partition as storage. If
>> the partition size is not rounded to 1 MB, the last write will fail with the
>> current code, and there is no way we can expand the partition.
>
> Actually, that you can change the image size is a special case. It only
> works on raw with file and sheepdog, and on qcow2 and qed. All other
> block drivers can't do it.
>
>>>> But I guess changing the protocol would be best as it would avoid
>>>> headaches to people who mistakenly created a file that is too small.
>>>
>>> We should think carefully before changing the protocol.
>>>
>>> Kevin?
>
> Can we do it in a compatible way? I agree that it would be nice to catch
> this error, but changing the protocol in an incompatible way for it
> seems to be too much.
No. However, it's not only about catching this error, but improving
the usability of block migration. I don't expect to change all at
once, I think it would be worthwhile to discuss if we want to improve
block migration.
Yoshi
> Anyway, it's independent of this patch and can be done on top.
>
> Kevin
>
>
