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.
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 have multiple servers that all store set members in a shared Redis cache. When the cache fills up, I need to persist the data to disk to free up RAM. I then plan to parse the dumped data such that I will be able to combine all of the values that belong to a given key in MongoDB.
My first plan was to have each server process attempt an sadd operation. If the request fails because Redis has reached maxmemory, I planned to query for each of my set keys, and write each to disk.
However, I am wondering if there is a way to use one of the inbuilt persistence methods in Redis to write the Redis data to disk and delete the key/value pairs after writing. If this is possible I could just parse the rdb dump and work with the data in that fashion. I'd be grateful for any help others can offer on this question.
Redis' persistence is meant to be used for whatever's in the RAM. Put differently, you can't persist what ain't in RAM.
To answer your question: no, you can't use persistence to "offload" data from RAM.
I understand redis can persist data, but during operation, when you look up a key on a redis server, does it ever need to go look for that key in the persisted data (files), or does it ONLY look for it in in-memory cache? What if the key is not found in the cache, does it automatically try to look for it on disk? How does it deal with cache being too big, if it writes the cache to memory, does it then clear the cache ?
All keys in Redis are always in memory. Data persisted to disk is only accessed during bootstrap for recovery purposes.
Redis is a in-memory data store. It can only hold a data set that can be fit into memory. Data in memory is NOT cache, but the whole data set. Redis can persist data on disk. However, the disk data is used for recovery, NOT for search. See the doc for details.
does it ever need to go look for that key in the persisted data (files), or does it ONLY look for it in in-memory cache?
NO. It never looks up a key in the persisted data.
What if the key is not found in the cache, does it automatically try to look for it on disk?
NO.
How does it deal with cache being too big, if it writes the cache to memory, does it then clear the cache ?
You can configure a policy to evict keys when the memory is insufficient. See the doc for details.
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.
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.