Re: [Qemu-devel] [PATCH] Fix block migration when the device size is not a multiple of 1 MB

[Pierre Riteau]

On 21 janv. 2011, at 15:30, Yoshiaki Tamura wrote:

> 2011/1/21 Pierre Riteau <address@hidden>:
>> On 21 janv. 2011, at 15:21, Yoshiaki Tamura wrote:
>> 
>>> 2011/1/21 Pierre Riteau <address@hidden>:
>>>> On 21 janv. 2011, at 14:59, Yoshiaki Tamura wrote:
>>>> 
>>>>> 2011/1/21 Pierre Riteau <address@hidden>:
>>>>>> On 21 janv. 2011, at 13:36, Yoshiaki Tamura wrote:
>>>>>> 
>>>>>>> 2011/1/21 Kevin Wolf <address@hidden>:
>>>>>>>> Am 21.01.2011 13:15, schrieb Yoshiaki Tamura:
>>>>>>>>> 2011/1/21 Pierre Riteau <address@hidden>:
>>>>>>>>>> Le 20 janv. 2011 à 17:18, Yoshiaki Tamura <address@hidden> a écrit :
>>>>>>>>>> 
>>>>>>>>>>> 2011/1/20 Pierre Riteau <address@hidden>:
>>>>>>>>>>>> On 20 janv. 2011, at 03:06, Yoshiaki Tamura wrote:
>>>>>>>>>>>> 
>>>>>>>>>>>>> 2011/1/19 Pierre Riteau <address@hidden>:
>>>>>>>>>>>>>> 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.
> 
> I thought supporting resize while block-migration would be a good
> feature because Kemari is live migrating again and again :)


Then bdrv_getlength would need to be called in the sender loop as well.

But there's one thing I don't know: how does the guest cope with online disk 
size changes? AFAIK Linux detects the size of the disk at boot.

-- 
Pierre Riteau -- PhD student, Myriads team, IRISA, Rennes, France
http://perso.univ-rennes1.fr/pierre.riteau/