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

[Fiona Ebner]

Am 29.02.24 um 12:48 schrieb Vladimir Sementsov-Ogievskiy:
> On 29.02.24 13:11, Fiona Ebner wrote:
>>
>> 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!
> 
> Aha understand.
> 
> For simplicity, let's consider case, when source "cluster size" = "job
> cluster size" = "bitmap granularity" = "target cluster size".
> 
> Which types of clusters we should consider, when we want to handle guest
> write?
> 
> 1. Clusters, that should be copied by background process
> 
> These are dirty clusters from user-given bitmap, or if we do a full-disk
> mirror, all clusters, not yet copied by background process.
> 
> For such clusters we simply ignore the unaligned write. We can even
> ignore the aligned write too: less disturbing the guest by delays.
> 

Since do_sync_target_write() currently doesn't ignore aligned writes, I
wouldn't change it. Of course they can count towards the "done_bitmap"
you propose below.

> 2. Clusters, already copied by background process during this mirror job
> and not dirtied by guest since this time.
> 
> For such clusters we are safe to do unaligned write, as target cluster
> must be allocated.
> 

Right.

> 3. Clusters, not marked initially by dirty bitmap.
> 
> What to do with them? We can't do unaligned write. I see two variants:
> 
> - do additional read from source, to fill the whole cluster, which seems
> a bit too heavy
> 

Yes, I'd rather only do that as a last resort.

> - just mark the cluster as dirty for background job. So we behave like
> in "background" mode. But why not? The maximum count of such "hacks" is
> limited to number of "clear" clusters at start of mirror job, which
> means that we don't seriously affect the convergence. Mirror is
> guaranteed to converge anyway. And the whole sense of "write-blocking"
> mode is to have a guaranteed convergence. What do you think?
> 

It could lead to a lot of flips between job->actively_synced == true and
== false. AFAIU, currently, we only switch back from true to false when
an error happens. While I don't see a concrete issue with it, at least
it might be unexpected to users, so it better be documented.

I'll try going with this approach, thanks!

> 
> ----
> 
> Of course, we can't distinguish 3 types by on dirty bitmap, so we need
> the second one. For example "done_bitmap", where we can mark clusters
> that were successfully copied. That would be a kind of block-status of
> target image. But using bitmap is a lot better than querying
> block-status from target.

Best Regards,
Fiona