Messages in this thread | | | Date | Wed, 25 Jan 2006 14:36:37 +0800 | From | Max Waterman <> | Subject | Re: io performance... |
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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?
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