I am new to redis, So please bear with me. Lets say I have configured a redis to have a maxmemory of 50mb, and I set eviction policy to allkeys-lru. And then I keep inserting and querying data. When the process memory gets to 50mb it starts to evict least recently used items.
My questions is do the evicted items persist on disk or are they lost for ever ? I mean if I do a GET for an evicted key, what do I get. Does redis fetch it from disk ?
Evicted is gone. With redis, nothing is on disk that isn't also in memory. (Technically, there will still probably be traces of it for some time, but that's just implementation details. As far as the data model is concerned, it's been deleted, and a GET won't find it.)
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Is there an advantage to set a default value for an entry that will be heavily queried in Redis or will querying for the unset key take the same time?
Given the keys are stored in a distributed hash, it will have to check that the key is not in the bucket before returning on a miss, which may be a bit slower than finding and stopping at a hit. Is the bucket sorted of linear? Does anything else make it slower either way?
Redis is setup in a cluster and has many million entries in this case.
I'm assuming you're just talking about strings & hashes here here (so the only operations you care about are set/get, maybe hget/hset) - From Redis' perspective, a cache hit and cache miss have the same time complexity, if anything, a cache miss will be faster because redis will not have to transfer any data back over the socket to your app.
I am experiencing a very strange case happen in production with redis, a large amount of redis keys are evicted unexpectedly even though memory not reach max configuration.
Current redis setting is max mem = 7GB, volatile-ttl.
Most of the keys are set a TTL when store to Redis.
Below graph showing a large drop in redis key eventhough memory at the time was only 3.5GB (<< 7GB)
According to my understanding, Redis would evict keys only when memory reach max-mem. And even when it does, it will only drop keys gradually according to the need for inserting new keys.
Thank you very much!
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 want to use Redis in this way
Load the entries (which are reading/editing by user) from file db,
set to expire in a period of time
Edit & read them on Redis
Store back to file db when it is auto-deleted by Redis (while it is
evicted to free memory or it is expired )
This isn't something that Redis does OOTB but with a little effort you can achieve it. See this for more info: https://stackoverflow.com/a/25827681/3160475