On 21 janv. 2011, at 15:21, Yoshiaki Tamura wrote:
> 2011/1/21 Pierre Riteau <pierre.rit...@irisa.fr>:
>> On 21 janv. 2011, at 14:59, Yoshiaki Tamura wrote:
>>
>>> 2011/1/21 Pierre Riteau <pierre.rit...@irisa.fr>:
>>>> On 21 janv. 2011, at 13:36, Yoshiaki Tamura wrote:
>>>>
>>>>> 2011/1/21 Kevin Wolf <kw...@redhat.com>:
>>>>>> Am 21.01.2011 13:15, schrieb Yoshiaki Tamura:
>>>>>>> 2011/1/21 Pierre Riteau <pierre.rit...@irisa.fr>:
>>>>>>>> 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.
>>>>>>>>
>>>>>>>
>>>>>>> Right. But in case of partition doesn't the check in the patch below
>>>>>>> return error? Does bdrv_getlength return the size correctly?
>>>>>>
>>>>>> I'm pretty sure that it does. We would have problems in other places if
>>>>>> it didn't (e.g. we're checking if I/O requests are within the disk size).
>>>>>
>>>>> Sorry for the noise. I just learned it's returning the value of lseek
>>>>> in case of raw-posix.
>>>>
>>>>
>>>> And it does a ioctl call on other platforms than Linux.
>>>
>>> Thanks. Just a quick question regarding total_sectors.
>>> BlockDriverState seems to contain total_sectors. Can we avoid
>>> calling bdrv_getlength() if bs->total_sectors were already there?
>>
>> From a comment in bdrv_getlength():
>>
>> Fixed size devices use the total_sectors value for speed instead of
>> issuing a length query (like lseek) on each call. Also, legacy block
>> drivers don't provide a bdrv_getlength function and must use
>> total_sectors.
>>
>> So using bdrv_getlength will protect against devices being resized during
>> migration, but as far as I can see, the sender side doesn't support it: the
>> value of total_sectors is cached for the whole block migration.
>
> Even if the sender supports it, as far as total_sectors isn't
> sent to the receiver, can we follow the resize on the receiver?
I was referring to the complex, and probably unrealistic scenario, where a user
allocates a file of the correct size on the receiving side, starts block
migration, and during migration grows the size of the disk on both the sender
and receiver side.
--
Pierre Riteau -- PhD student, Myriads team, IRISA, Rennes, France
http://perso.univ-rennes1.fr/pierre.riteau/
