Very low disk performance on 5.x

[Poul-Henning Kamp]

In message <42762AD8.1020607 at centtech.com>, Eric Anderson writes:

>If I write a 10GB file to disk (RAID array has 1GB cache, system has 1GB 
>memory), then I should definitely see better read performance reading 
>that same file back to /dev/null than writing it, right?

Nope, quite the contrary: you will get much better write performance
because your write operation finishes the moment it has hit the cache,
but your read request only comes fast if it happens to be in the cache.

With a 1GB cache and a 10GB file, this clearly will not be the case
unless some kind of very good read-ahead happens in the RAID5 unit.

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