Snapshotting is not very durable. If your computer running Redis stops, your power line fails, or you accidentally kill -9 your instance, the latest data written on Redis will get lost. While this may not be a big deal for some applications, there are use cases for full durability, and in these cases Redis was not a viable option.
The append-only file is an alternative, fully-durable strategy for Redis. It became available in version 1. From now on, every time Redis receives a command that changes the dataset e. When you restart Redis it will re-play the AOF to rebuild the state.
As you can guess, the AOF gets bigger and bigger as write operations are performed. For example, if you are incrementing a counter times, you'll end up with a single key in your dataset containing the final value, but entries in your AOF. So Redis supports an interesting feature: it is able to rebuild the AOF in the background without interrupting service to clients.
If you're using the AOF with Redis 2. Redis 2. You can configure how many times Redis will fsync data on disk. There are three options:. The suggested and default policy is to fsync every second. It is both very fast and pretty safe. The always policy is very slow in practice, but it supports group commit, so if there are multiple parallel writes Redis will try to perform a single fsync operation. It is possible that the server crashed while writing the AOF file, or that the volume where the AOF file is stored was full at the time of writing.
When this happens the AOF still contains consistent data representing a given point-in-time version of the dataset that may be old up to one second with the default AOF fsync policy , but the last command in the AOF could be truncated. The latest major versions of Redis will be able to load the AOF anyway, just discarding the last non well formed command in the file. In this case the server will emit a log like the following:.
You can change the default configuration to force Redis to stop in such cases if you want, but the default configuration is to continue regardless the fact the last command in the file is not well-formed, in order to guarantee availability after a restart. If the AOF file is not just truncated, but corrupted with invalid byte sequences in the middle, things are more complex. Redis will complain at startup and will abort:.
The best thing to do is to run the redis-check-aof utility, initially without the --fix option, then understand the problem, jump at the given offset in the file, and see if it is possible to manually repair the file: the AOF uses the same format of the Redis protocol and is quite simple to fix manually. Otherwise it is possible to let the utility fix the file for us, but in that case all the AOF portion from the invalid part to the end of the file may be discarded, leading to a massive amount of data loss if the corruption happened to be in the initial part of the file.
Log rewriting uses the same copy-on-write trick already in use for snapshotting. This is how it works:. Redis forks , so now we have a child and a parent process. The parent accumulates all the new changes in an in-memory buffer but at the same time it writes the new changes in the old append-only file, so if the rewriting fails, we are safe. When the child is done rewriting the file, the parent gets a signal, and appends the in-memory buffer at the end of the file generated by the child.
Now Redis atomically renames the old file into the new one, and starts appending new data into the new file. There is a different procedure to do this in Redis 2. In order to do so Redis will block to generate the initial dump, then will open the file for writing, and will start appending all the next write queries.
This is optional, if you wish you can take both the persistence methods enabled. Before starting this section, make sure to read the following sentence: Make Sure to Backup Your Database.
Redis is very data backup friendly since you can copy RDB files while the database is running: the RDB is never modified once produced, and while it gets produced it uses a temporary name and is renamed into its final destination atomically using rename 2 only when the new snapshot is complete. This means that copying the RDB file is completely safe while the server is running. This is what we suggest:. The file may lack the final part but Redis will be still able to load it see the previous sections about truncated AOF files.
Disaster recovery in the context of Redis is basically the same story as backups, plus the ability to transfer those backups in many different external data centers. This way data is secured even in the case of some catastrophic event affecting the main data center where Redis is running and producing its snapshots. More Redis Resources.
Xuewei Whitelaw Pundit. How do you persist data? Persistence is "the continuance of an effect after its cause is removed". In the context of storing data in a computer system, this means that the data survives after the process with which it was created has ended. In other words, for a data store to be considered persistent, it must write to non-volatile storage.
Faye Gosto Pundit. Which is a difference between Memcached and Redis? Redis and Memcached are both in-memory data storage systems. Memcached is a high-performance distributed memory cache service, and Redis is an open-source key-value store.
Similar to Memcached , Redis stores most of the data in the memory. In this article, we will examine the difference between Redis and Memcached. Shama Raoui Pundit. Is used to flush the transaction queue and exit from the transaction? Instead of executing these commands, Redis will queue them. All the commands are executed once EXEC is called. Bikendi Heitzelmann Teacher.
How do I stop Redis server? Try killall redis - server. Manar Rosillo Supporter. What happens when Redis runs out of memory? What happens if Redis runs out of memory? If this limit is reached Redis will start to reply with an error to write commands but will continue to accept read-only commands , or you can configure it to evict keys when the max memory limit is reached in the case where you are using Redis for caching.
Taoufik Lukowia Supporter. What port is Redis running on? Nicereta El Mrini Supporter. How do I find my Redis password? The Redis password is stored inside the redis. Estela Cardosa Beginner. How do I start Redis on a different port? Locate your redis. Copy the file or edit that one and change the port directive to any free port. Active 3 years, 9 months ago. Viewed 9k times. Improve this question.
Jay Jung Jay Jung 1, 16 16 silver badges 41 41 bronze badges. Add a comment. Active Oldest Votes. Where does it belong? Wherever you want. Is it possible to change to location of dump. Yes, it is possible. There two possible ways I can think of. To reload previous data, previous dump. That's it! But this way is not permanent because server restart will use the old directory.
Remember, redis server has to have write permission to that file. Improve this answer. Does this change in a production environment?
Homer Updated my answer. Akash Dathan Akash Dathan 3, 1 1 gold badge 22 22 silver badges 41 41 bronze badges.
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