Re: raid1 does not seem faster

[Bill Davidsen <davidsen@tmr.com>]

Justin Piszcz wrote:
>
>
> On Thu, 5 Apr 2007, Iustin Pop wrote:
>
>> On Thu, Apr 05, 2007 at 04:11:35AM -0400, Justin Piszcz wrote:
>>>
>>>
>>> On Thu, 5 Apr 2007, Iustin Pop wrote:
>>>
>>>> On Wed, Apr 04, 2007 at 07:11:50PM -0400, Bill Davidsen wrote:
>>>>> You are correct, but I think if an optimization were to be done, some
>>>>> balance between the read time, seek time, and read size could be 
>>>>> done.
>>>>> Using more than one drive only makes sense when the read transfer 
>>>>> time is
>>>>> significantly longer than the seek time. With an aggressive 
>>>>> readahead set
>>>>> for the array that would happen regularly.
>>>>>
>>>>> It's possible, it just takes the time to do it, like many other 
>>>>> "nice"
>>>>> things.
>>>>
>>>> Maybe yes, but why optimise the single-reader case? raid1 already can
>>>> read in parallel from the drives when multiple processes read from the
>>>> raid1. Optimising the single reader can help in hdparm or other
>>>> benchmark cases, but in real life I see very often the total 
>>>> throughput
>>>> of a (two drive) raid1 being around two times the throughput of a 
>>>> single
>>>> drive.
>>>>
>>>> regards,
>>>> iustin
>>>> -
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>>>
>>> Really? I have copied a file from a SW RAID1 (5GB) and I only saw
>>> 60MB/s not the 120MB/s the (RAID1) is capable of to the
>>> destination (which can easily do > 160MB/s sustained read/write).
>>
>> Did you copy it multi-threaded? I said "*multiple-readers* show improved
>> speed" and you said "I copied *one* file". Try copying two files in
>> parallel.
>>
>> I'm doing in two xterms "cat file1 >/dev/null", "cat file2 >/dev/null"
>> and my raid1 shows ~110 MB/s, each drive doing about half.  On file only
>> does about 60 MB/s (this is over a PCI raid controller so the max 110
>> MB/s is a PCI bus limitation).
>>
>> Iustin
>> -
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>>
>
> Ah, nope, I only did a single copy, I did not parallelize the operations.
And it would be nice not to have to jump through hoops to get better 
performance... You can implement raid[01] at the user level, too, why 
bother to have it in the kernel? ;-)

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979