On 06/05/2014 09:01 AM, Haomai Wang wrote:
Hi, Previously I sent a mail about the difficult of rbd snapshot size statistic. The main solution is using object map to store the changes. The problem is we can't handle with multi client concurrent modify. Lack of object map(like pointer map in qcow2), it cause many problems in librbd. Such as clone depth, the deep clone depth will cause remarkable latency. Usually each clone wrap will increase two times of latency. I consider to make a tradeoff between multi-client support and single-client support for librbd. In practice, most of the volumes/images are used by VM, there only exist one client will access/modify image. We can't only want to make shared image possible but make most of use cases bad. So we can add a new flag called "shared" when creating image. If "shared" is false, librbd will maintain a object map for each image. The object map is considered to durable, each image_close call will store the map into rados. If the client is crashed and failed to dump the object map, the next client open the image will think the object map as out of date and reset the objectmap.
Why not flush out the object map every X period? Assume a client runs for weeks or months and you would keep that map in memory all the time since the image is never closed.
We can easily find the advantage of this feature: 1. Avoid clone performance problem 2. Make snapshot statistic possible 3. Improve librbd operation performance including read, copy-on-write operation. What do you think above? More feedbacks are appreciate!
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