- Author(s): Zhexuan Yang
- Last updated: 2018/09/09
- Discussion at: Not applicable
When it comes to time-related calculation, it is hard for the distributed system. This proposal tries to resolve two problems: 1. timezone may be inconsistent across multiple TiDB instances, 2. performance degradation caused by pushing System down to TiKV. The impact of this proposal is changing the way of TiDB inferring system's timezone name. Before this proposal, the default timezone name pushed down to TiKV is System when session's timezone is not set. After this, TiDB evaluates system's timezone name via TZ environment variable and the path of the soft link of /etc/localtime. If both of them are failed, TiDB then push UTC to TiKV.
After we solved the daylight saving time issue, we found the performance degradation of TiKV side. Thanks for the investigation done by engineers from TiKV. The root cause of such performance degradation is that TiKV infers System timezone name via a third party lib, which calls a syscall and costs a lot. In our internal benchmark system, after this PR, our codebase is 1000 times slower than before. We have to address this.
Another problem needs also to be addressed is the potentially incosistent timezone name across multiple TiDB instances. TiDB instances may reside at different timezone which could cause incorrect calculation when it comes to time-related calculation. Just getting TiDB's system timezone could be broken. We need find a way to ensure the uniqueness of global timezone name across multiple TiDB's timezone name and also to leverage to resolve the performance degradation.
Firstly, we need to introduce the TZ environment. In POSIX system, the value of TZ variable can be one of the following three formats. A detailed description can be found in this link
* std offset
* std offset dst [offset], start[/time], end[/time]
* :characters
The std means the IANA timezone name; the offset means timezone offset; the dst indicates the leading timezone having daylight saving time.
In our case, which means both TiDB and TiKV, we need care the first and third formats. For answering why we do not need the second format, we need to review how Golang evaluates timezone. In time package, the method LoadLocation reads tzData from pre-specified sources(directories may contain tzData) and then builds time.Location from such tzData which already contains daylight saving time information.
In this proposal, we suggest setting TZ to a valid IANA timezone name which can be read from TiDB later. If TiDB can't get TZ or the supply of TZ is invalid, TiDB just falls back to evaluate the path of the soft link of /etc/localtime. In addition, a warning message telling the user you should set TZ properly will be printed. Setting TZ can be done in our tidb-ansible project, it is also can be done at user side by export TZ="Asia/Shanghai". If both of them are failed, TiDB will use UTC as timezone name.
The positive side of this change is resolving performance degradation issue and ensuring the uniqueness of global timezone name in multiple TiDB instances.
The negative side is just adding a config item which is a very small matter and the user probably does not care it if we can take care of it and more importantly guarantee the correctness.
We tried to read system timezone name by checking the path of the soft link of /etc/localtime but, sadly, failed at a corner case. The failed case is docker. In docker image, it copies the real timezone file and links to /usr/share/zoneinfo/utc. The timezone data is correct but the path is not. Regarding of UTC, Golang just returns UTC instance and will not further read tzdata from sources. This leads to a fallback solution. When we cannot evaluate from the path, we fall back to UTC.
It does not have compatibility issue as long as the user deploys by tidb-ansible. We may mention this in our release-node and the message printed before tidb quits, which must be easy to understand.
The upgrading process need to be handled in particular. TZ environment variable has to be set before we start new TiDB binary. In this way, the following bootstrap process can benefit from this and avoid any hazard happening.
The implementation is relatively easy. We just get TZ environment from system and check whether it is valid or not. If it is invalid, TiDB evaluates the path of soft link of /etc/localtime. In addition, a warning message needs to be printed indicating user has to set TZ variable properly. For example, if /etc/localtime links to /usr/share/zoneinfo/Asia/Shanghai, then timezone name TiDB gets should be Asia/Shanghai.
In order to ensure the uniqueness of global timezone across multiple TiDB instances, we need to write timezone name into variable_value with variable name system_tz in mysql.tidb. This cached value can be read once TiDB finishes its bootstrap stage. A method loadLocalStr can do this job.
PR of this proposal: https://github.com/pingcap/tidb/pull/7638/files PR of change TZ loading logic of golang: golang/go#27570