Redis is a database in-memory but persistent on disk meanwhile.
Q1: So I wonder does this mean that when redis server starts, it will automatically load all the data on the disk into memory?
Q2: And when writing data to redis, will it both update in the memory and the disk?
Can anyone please help me answer my two questions?
Q1: So I wonder does this mean that when redis server starts, it will
automatically load all the data on the disk into memory?
Yes, depending on the configuration, Redis performs snapshots of memory to disk and, when Redis is restarted it can take latest snapshot and take it to memory again automatically.
Q2: And when writing data to redis, will it both update in the memory
and the disk?
Redis prioritizes writes on memory and writes to disk are done in a separate thread. The answer then is yes, it writes data to both memory and disk, but it might happen that a server failure may produce a data loss since it's not mandatory to Redis to persist data to disk.
Check official docs about persistence to learn more about the topic.
Related
I want to understand when a cache is created with native persistence enabled, will it store the data in the defined data region/RAM and in the disk at the same time? Is there any way I can restrict the disk utilization for storing the data?
Additionally, in a cluster of 3 due to any reason the disk got full for one of the nodes and there is not enough memory available, what will be the impact on the cluster?
Yes, data will be stored both in RAM and on the disk. I does not have to fit in RAM at the same time.
If you run out of disk space, your persistent store will likely be corrupted.
I might be wrong but still asking this question. ;-)
So I am planning to use redis as a persistent storage(Primary Storage). I am having AOF enabled.I know redis will load this data during server start up. Let us say I have 10GB data and 5 GB ram, If I try to search for a key which is not loaded in RAM, will it check AOF and load that data to RAM by offloading any unused keys?
You cannot have less memory than data size in Redis. In your example Redis would run out of memory during start-up. You find more answers here: http://redis.io/topics/faq
I am facing some scaling issues with my redis instances and was wondering if there's a way to configure redis to save data only to disk (and not hold it in memory). That way I could just increase disk space and not RAM.
Right now my instances are getting stuck and just hang when they reach the memory limit.
Thanks!
No - Redis, atm, is an in-memory database. That means that all data that it manages resides first and foremost in RAM.
Based on the
http://redis.io/topics/faq
Redis is an in-memory but persistent on disk database.
So may I know redis save key/value in memory or in disk? or both?
When writing value in Redis, it write into memory and disk at the same time?
Thanks for the concept.
depending on how you configure it, redis can periodically back up the existing state to disk, but otherwise, everything is in memory.
Redis will atomically snapshot its memory state to disk if so configured. See this part of the docs for more info:
http://redis.io/topics/persistence
So you can have different levels of durability. For the most part, when you get a key, it is out of memory and when you set a key it is also in memory. The data is written to disk independently of read/write operations.
Is Redis memory only store like memcached or does it write the data to the disk? If it does write to the disk, how often is the disk written to?
Redis persistence is described in detail here:
http://redis.io/topics/persistence
By default, redis performs snapshotting:
By default Redis saves snapshots of the dataset on disk, in a binary file called dump.rdb. You can configure Redis to have it save the dataset every N seconds if there are at least M changes in the dataset, or you can manually call the SAVE or BGSAVE commands.
For example, this configuration will make Redis automatically dump the dataset to disk every 60 seconds if at least 1000 keys changed: save 60 1000
Another good reference is this link to the author's blog where he tries to explain how redis persistance works:
http://antirez.com/post/redis-persistence-demystified.html
Redis holds all data in memory. If the size of an application's data is too large for that, then Redis is not an appropriate solution.
However, Redis also offers two ways to make the data persistent:
1) Snapshots at predefined intervals, which may also depend on the number of changes. Any changes between these intervals will be lost at a power failure or crash.
2) Writing a kind of change log at every data change. You can fine-tune how often this is physically written to the disk, but if you chose to always write immediately (which will cost you some performance), then there will be no data loss caused by the in-memory nature of Redis.