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

[<tang.junhui@zte.com.cn>]

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Hello Martin,





This exmaple shows why we need  to stop merging,

Yes,  It's fine since we have "stop merging" in current code.




RegardsTang












原始邮件



发件人： ＜mwilck@suse.com＞
收件人： ＜dm-devel@redhat.com＞
日 期 ：2017年02月20日 19:24
主 题 ：Re: [dm-devel] 答复: Re:  [PATCH] multipath-tools: improve processing efficiency for addition and deletion of multipath devices





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@suse.com＞, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)

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