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SubjectRe: io performance...
Ian Soboroff wrote:
> Max Waterman <davidmaxwaterman+kernel@fastmail.co.uk> writes:
>
>> Phillip Susi wrote:
>>> Right, the kernel does not know how many disks are in the array, so
>>> it can't automatically increase the readahead. I'd say increasing
>>> the readahead manually should solve your throughput issues.
>> Any guesses for a good number?
>>
>> We're in RAID10 (2+2) at the moment on 2.6.8-smp. These are the block
>> numbers I'm getting using bonnie++ :
>>
>> [...]
>> We're still wondering why rd performance is so low - seems to be the
>> same as a single drive. RAID10 should be the same performance as RAID0
>> over two drives, shouldn't it?
>
> I think bonnie++ measures accesses to many small files (INN-like
> simulation) and database accesses. These are random accesses, which
> is the worst access pattern for RAID. Seek time in a RAID equals the
> longest of all the drives in the RAID, rather than the average. So
> bonnie++ is domninated by your seek time.

You think so? I had assumed when bonnie++'s output said 'sequential
access' that it was the opposite of random, for example (raid0 on 5
drives) :

> +---------------------------------------------------------------------------------------------------------------------------------------------------+
> | |Sequential Output |Sequential Input | | |Sequential Create |Random Create |
> |---------------------+------------------------------+--------------------|Random |-----+----------------------------+----------------------------|
> | |Size:Chunk|Per Char |Block |Rewrite |Per Char |Block |Seeks |Num |Create |Read |Delete |Create |Read |Delete |
> | |Size | | | | | | |Files| | | | | | |
> |---------------------+---------+----------+---------+---------+----------+---------+-----+--------+---------+---------+--------+---------+---------|
> | |K/sec|% |K/sec |% |K/sec|% |K/sec|% |K/sec |% |/ sec|% | |/ |% |/ sec|% |/ sec|% |/ |% |/ sec|% |/ sec|% |
> | | |CPU| |CPU| |CPU| |CPU| |CPU| |CPU| |sec |CPU| |CPU| |CPU|sec |CPU| |CPU| |CPU|
> |---------------------+-----+---+------+---+-----+---+-----+---+------+---+-----+---+-----+----+---+-----+---+-----+---+----+---+-----+---+-----+---|
> |hostname |2G |48024|96 |121412|13 |59714|10 |47844|95 |200264|21 |942.8|1 |16 |4146|99 |+++++|+++|+++++|+++|4167|99 |+++++|+++|14292|99 |
> +---------------------------------------------------------------------------------------------------------------------------------------------------+

Am I wrong? If so, what exactly does 'Sequential' mean in this context?

Max.
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