[dm-devel] 答复: Re: [PATCH] multipath-tools: improve processing efficiency for addition and deletion of multipath devices

[Martin Wilck]

Hello Tang,

＞ In the second case, we know all events in the sequence have the
> same
> ＞ WWID; in this case I think it would be safe to filter away "remove"
> ＞ events by subsequent "add" events, ending up with "add path1| add
> ＞ path2| remove path3". But I may be overlooking something here.
> 
> ＞ The dangerous thing if you have simultaneous remove and add events
> for
> ＞ the same LUN is that processing the "add" events is likely to fail
> in
> ＞ domap(). If you get "add path1 | remove path2", once you process
> "add
> ＞ path1", "path2" may not exist in the kernel any more, and "domap"
> will
> ＞ fail if you try to set up both; you may end up removing the map
> ＞ completely. IMHO the only safe way to process events in this
> situation
> ＞ is to merge the events into a single domap() call.
> 
> "case 2: remove path1 |add path1 |remove path2 |add path2 |remove
> path3"
> Since the mergering is starting from the latest to the eariest
> uevent,  so the
> uevent "remove path3" try to merger other uevents, if we do not stop
> mergering, 
> uevent "remove path3" would merge  uevent "remove path2" and uevent 
> "remove path1". Uevent "add  path2" would merge "add  path1". 
> The result is that we get two merged uevents:
> 1) add path2, path1
> 2) remove path3, path2, path1 

> Then we process the merged uevents from the earlier to later, We try
> to ”add 
> path2 path1“, and we would create or reload a map device with path1
> and path2. 
> Next we process ” remove path3, path2, path1“, so path1, path2 and
> path3 are 
> all removed from the map device, this is wrong,  the correct result
> is that the path1
> and path2 should be still in the map device.

I don't understand that. This would mean that you'd re-order "remove
path1 | add path1" into "add path1| remove path1", and the same for
path2. That may obviously never happen. Your "stop merging" logic
forbids it, and that's fine for me for now.

Regards
Martin

-- 
Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
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