Very low disk performance on 5.x

[Steven Hartland]

----- Original Message ----- 
From: "Poul-Henning Kamp" <phk at phk.freebsd.dk>
>>Don't mean to be terse here, but I'm talking about the same test done an 
>>two different RAID5 configurations, with different disks, and not just 
>>me - other users in this very thread see the same issue..
> 
> Uhm, if you are using RAID5 and your requests are not aligned and
> sized after the RAID5 you should *expect* read performance to be poor.
> 
> If you your request ends up accessing two different blocks even just
> once per stripe, this totally kills performance.

Interesting stuff so:
1. How to we test if this is happening?
2. How do we prevent it from happening?
3. Why would this be effecting reads and not writes as surely the same
blocking is being done for both?

RAID5 is becoming more and more the norm with disk and controller
prices dropping and as such I think it would be good if could get this.
one sorted.

    Steve


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