Redis why zset can't auto conver from skiplist coding to ziplist coding

zset-max-ziplist-entries 512 for example keyA, on start , keyA's value length < 512 so it's encoding is ziplist i go on call zadd, then keyA's coding convert to skiplist. Then i delete som items of until zcard keyA below 400. I find that the keyA's encoding is skiplist as before. Why? why zcard < 512 already, the encoding should convert to ziplist?

                                                                                       thank you very much!

Comment From: itamarhaber

This is by design - the conversion is one-way and Redis does not re-encode the skiplist to ziplist if the threshold is crossed on the way down.

Comment From: lygn128

ok, can you tell me the design philosophy in this？ and if I add the codes as follow. Which problems will result in of this? thank you very much. if (zzlLength(zobj->ptr) <= server.zset_max_ziplist_entries) zsetConvert(zobj,OBJ_ENCODING_ZIPLIST); if (sdslen(ele->ptr) <= server.zset_max_ziplist_value) zsetConvert(zobj,OBJ_ENCODING_ZIPPLIST);

Comment From: itamarhaber

The philosophy isn't mine but trying to reverse-engineer it I'd say that scaling-in is less of a concern because: 1. Constant-sized and growing data structures represent the majority of use cases, whereas "shrinking" ones are much rarer. 2. The ability to back-and-forth between encodings may result in pathological cases where the same structures thrash while tittering on the threshold. 3. We're against complexity ;)

On Tue, Aug 2, 2016 at 6:17 AM, lygn128 notifications@github.com wrote:

ok, can you tell me the design philosophy in this？ and if I add the codes as follow. Which problems will result in of this? thank you very much.

if (zzlLength(zobj->ptr) <= server.zset_max_ziplist_entries) zsetConvert(zobj,OBJ_ENCODING_ZIPLIST); if (sdslen(ele->ptr) <= server.zset_max_ziplist_value) zsetConvert(zobj,OBJ_ENCODING_ZIPPLIST);

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Comment From: itamarhaber

Closing as answered.