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