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If I list 4 databases like below with their sizes, does media being 2.5TB have any effect on the performance of queries on other databases? Does the number of databases have any effect on MySQL performance (without considering any usage on any databases)?

ads (259 MB)
media (2.5 TB)
forums (25 GB)
archives (853 MB)

So will SELECT * FROM archives take longer to run if media is 4 TB's than if it is 1 TB? Do indexes, number of tables, etc. have any effect cross database?

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its all about spindles, so if you have 5 disks making up 4TB and only 1 for the 1TB the 4TB should be faster. – tony roth Aug 6 '10 at 20:13
I'm speaking more so about the load on the MySQL server than how fast the hard drives can find the data – Webnet Aug 6 '10 at 20:23
well if you have a slower drives the cpu's will be less utilized! – tony roth Aug 6 '10 at 20:43
up vote 2 down vote accepted

I think other posters are misinterpreting your question.

To answer it: No - The size of one database does not impact on the speed of look-ups on another database sitting on the same instance. Your queries would slow down only if your query was referring to that other, large DB in some manner.

There are some subtler knock-on effects though, as soon as you do start to look at usage (I realize you excluded it). If your 'media' database is being accessed in a separate query, then that slower query and your faster query are still competing for a finite amount of resources. So indirectly, the larger, slower 'media' database could actually slow down a look-up on one of the others.

There's no direct bearing, though.

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If concurrent queries are being executed say, select * from media and select * from ads, I'm quite sure (hardware setup may impact this) they would end up competing for resources to execute the two queries and therefore impact query speed. – Tyler K Aug 7 '10 at 6:20
Explicitly stated in the question "Without considering any usage on the databases), and covered in the second paragraph of my answer. – Chris Thorpe Aug 7 '10 at 7:06

Performance is going to depend a lot on how much RAM you have and how much tuning you've done to mysql. If you've got a lot of RAM and have told MySQL that it can use it for extra buffers and the like, you'll get better performance. The best way to do such tuning is going to depend on the kind of load.

Performance of one database, however, is only going to be impacted by the size of other databases if those databases are being queried; if media is just sitting on the disk and not being queried at all, then it's not going to impact queries on the archives database. If, however, media is being queried, then it's going to start contending with the archives database's needs for RAM and CPU.

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