Very low disk performance on 5.x

[Eric Anderson]

Poul-Henning Kamp wrote:
> In message <4276281C.6060209 at centtech.com>, Eric Anderson writes:
> 
>>Poul-Henning Kamp wrote:
>>
>>>In message <427626DC.5030702 at centtech.com>, Eric Anderson writes:
>>>
>>>
>>>
>>>>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.
>>
>>Wouldn't this be a problem for writes then too?
> 
> 
> I presume you would only compare read to write performance on a RAID5
> device which has battery backed cache.
> 
> Without a battery backed cache (or pretending to have one) RAID5
> write performance is abysmall no matter which alignment you use.

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?

How about this - you tell me what test to run, and I'll do it (as long 
as it doesn't destory my data).

Eric



-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
A lost ounce of gold may be found, a lost moment of time never.
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