[ https://issues.jboss.org/browse/JGRP-1982?page=com.atlassian.jira.plugin.... ] Bela Ban edited comment on JGRP-1982 at 1/8/16 6:45 AM: -------------------------------------------------------- Turns out that compaction 'in the middle' doesn't work with the current design. If we have ||0||1||2||3||4||5||6||7|| |-|-|-|-|-|21|22|23| where low=21 and high=24. Compacting this yields: ||0||1||2||3|| |-|21|22|23| (low=21, high=24) However, growing the array again yields: ||0||1||2||3||4||5||6||7|| |-|21|22|23|-|-|-|-| (low=21, high=24) The problem here is that we hash a _seqno_ to an _index_, but once we've done this, we don't know what the actual seqno is, and hash on the indices only. E.g. indices for 21,22,23 and 5,6,7, then 1,2,3. The root issue is that a mod operation (hash) is not bijective, ie. {{21 % 8 = 5}}, but we cannot compute the original seqno 21 from 5. We'd have to store the original seqno with the value, but that's too much overhead... was (Author: belaban): Turns out that compaction 'in the middle' doesn't work with the current design. If we have ||0||1||2||3||4||5||6||7|| |-|-|-|-|-|21|22|23| where low=21 and high=24. Compacting this yield: ||0||1||2||3|| |-|21|22|23| (low=21, high=24) However, growing the array again yields: ||0||1||2||3||4||5||6||7|| |-|21|22|23|-|-|-|-| (low=21, high=24) The problem here is that we hash a _seqno_ to an _index_, but once we've done this, we don't know what the actual seqno is, and hash on the indices only. E.g. indices for 21,22,23 and 5,6,7, then 1,2,3. The root issue is that a mod operation (hash) is not bijective, ie. {{21 % 8 = 5}}, but we cannot compute the original seqno 21 from 5. We'd have to store the original seqno with the value, but that's too much overhead... -- This message was sent by Atlassian JIRA (v6.4.11#64026)