Re: io performance...

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Ian Soboroff wrote:
Max Waterman <[email protected]> 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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