What is difference MongoDB wiredTiger cache with in-memory DB

Question

As i know MonogoDB cache working set in RAM.

Then if i increase wiredTigerCacheSizeGB as much as all of data in disk, does it work as fast as in-memory db?

if no, what is difference?

Answer 1

See In-Memory Storage Engine and WiredTiger Storage Engine

(In-memory) By avoiding disk I/O, the in-memory storage engine allows for more predictable latency of database operations.

Keep in mind that you are limited a 10000 GB when setting wiredTigerCacheSizeGB. You should also disable journaling and set storage.syncPeriodSecs to 0 in order to increase performance of WiredTiger. But, still WiredTiger has to create WiredTiger.wt and WiredTiger.turtle at least...

PS. I think this link might answer your question

Answer 2

I cannot answer all your questions.

A cache reads data from disk and keeps it in the RAM. When you access such data again then you read it from RAM instead of reading it again from disk - which would be much slower.

So, a cache is useless if you have to read the data only once. Some applications anticipate the data you may read in future and put it into the cache in advance.

The MongoDB in-memory DB puts all data into RAM only, it does not read or write anything from disk, apart from some logging data. When you stop an in-memory MongoDB process then all data is lost.

The wiredTiger storage engine is a data format used by MongoDB to store data persistently on disk.

Answer 3

If you set wiredTigerCacheSizeGB high enough to hold all of your data, then all of your reads will be satisfied from the cache. Writes will update the cache and also be written to storage.

If you use the in-memory configuration then all of your reads will be satisfied from memory. Writes will only go to memory and will not be stored on disk.

So if your workload is mostly reads, then the large cache will behave similarly to an in-memory DB. If your workload has a lot of writes, then the large cache configuration may be slower because it needs to write to disk.

Also, the in-memory DB will not preserve your data in the event of a crash, since it only holds data in memory.