Re: [RFC 0/4] mirror: implement incremental and bitmap modes

[Fiona Ebner]

Am 28.02.24 um 17:06 schrieb Vladimir Sementsov-Ogievskiy:
> On 28.02.24 19:00, Vladimir Sementsov-Ogievskiy wrote:
>> On 16.02.24 13:55, Fiona Ebner wrote:
>>> Now, the IO test added in patch 4/4 actually contains yet another use
>>> case, namely doing incremental mirrors to stand-alone qcow2 "diff"
>>> images, that only contain the delta and can be rebased later. I had to
>>> adapt the IO test, because its output expected the mirror bitmap to
>>> still be dirty, but nowadays the mirror is apparently already done
>>> when the bitmaps are queried. So I thought, I'll just use
>>> 'write-blocking' mode to avoid any potential timing issues.
>>>
>>> But this exposed an issue with the diff image approach. If a write is
>>> not aligned to the granularity of the mirror target, then rebasing the
>>> diff image onto a backing image will not yield the desired result,
>>> because the full cluster is considered to be allocated and will "hide"
>>> some part of the base/backing image. The failure can be seen by either
>>> using 'write-blocking' mode in the IO test or setting the (bitmap)
>>> granularity to 32 KiB rather than the current 64 KiB.
>>>
>>> The question is how to deal with these edge cases? Some possibilities
>>> that would make sense to me:
>>>
>>> For 'background' mode:
>>> * prohibit if target's cluster size is larger than the bitmap
>>>    granularity
>>> * document the limitation
>>>
>>> For 'write-blocking' mode:
>>> * disallow in combination with bitmap mode (would not be happy about
>>>    it, because I'd like to use this without diff images)
>>
>> why not just require the same: bitmap granularity must be >= target
>> granularity
>>

For the iotest's use-case, that only works for background mode. I'll
explain below.

>>> * for writes that are not aligned to the target's cluster size, read
>>>    the relevant/missing parts from the source image to be able to write
>>>    whole target clusters (seems rather complex)
>>
>> There is another approach: consider and unaligned part of the request,
>> fit in one cluster (we can always split any request to "aligned"
>> middle part, and at most two small "unligned" parts, each fit into one
>> cluster).
>>
>> We have two possibilities:
>>
>> 1. the cluster is dirty (marked dirty in the bitmap used by background
>> process)
>>
>> We can simply ignore this part and rely on background process. This
>> will not affect the convergence of the mirror job.
>>

Agreed.

>> 2. the cluster is clear (i.e. background process, or some previous
>> write already copied it)
>>

The iotest creates a new target image for each incremental sync which
only records the diff relative to the previous mirror and those diff
images are later rebased onto each other to get the full picture.

Thus, it can be that a previous mirror job (not just background process
or previous write) already copied a cluster, and in particular, copied
it to a different target!

>> In this case, we are safe to do unaligned write, as target cluster
>> must be allocated.

Because the diff image is new, the target's cluster is not necessarily
allocated. When using write-blocking and a write of, e.g., 9 bytes to a
clear source cluster comes in, only those 9 bytes are written to the
target. Now the target's cluster is allocated but with only those 9
bytes of data. When rebasing, the previously copied cluster is "masked"
and when reading the rebased image, we only see the cluster with those 9
bytes (and IIRC, zeroes for the rest of the cluster rather than the
previously copied data).

>>
>> (for bitmap-mode, I don't consider here clusters that are clear from
>> the start, which we shouldn't copy in any case)
>>

We do need to copy new writes to any cluster, and with a clear cluster
and write-blocking, the issue can manifest.

> 
> Hmm, right, and that's exactly the logic we already have in
> do_sync_target_write(). So that's enough just to require that
> bitmap_granularity >= target_granularity
> 

Best Regards,
Fiona