Kay Diederichs wrote:
> Chan Chung Hang Christopher schrieb:
>
>>> "md1 will read from both disk" is not true in general.
>>> RAID1 md reads from one disk only; it uses the other one in case the
>>> first one fails. No performance gain from multiple copies.
>>>
>>>
>> I beg to differ. I have disks in a raid1 md array and iostat -x 1 will
>> show reads coming off both disks. Unless you do not have the multipath
>>
>
> look more carefully - with the current 2.6.18-9.1.22 kernel the bulk of
> the data are read from one of the disks
>
>
Hmm...right now I do not have a Centos 5 box handy. Come on you chums
who have blasted me before about multipath. Prove him wrong with data
please. I can only pull evidence off a Hardy box.
>> module loaded, md will read off both disks. Now whether md will read
>> equally off both disks, that certainly will not be true in general.
>>
>>> You can easily see this for yourself by setting up a RAID1 from e.g.
>>> sda1 and sdb1 - /proc/mdstat is:
>>>
>>> Personalities : [raid1]
>>> md1 : active raid1 sdb1[1] sda1[0]
>>> 104320 blocks [2/2] [UU]
>>>
>>> and then comparing the output of hdparm -tT :
>>>
>>>
>> ROTFL.
>>
>> How about using the proper tool (iostat) and generating some disk load
>> instead?
>>
>
> hdparm -tT tests one type of disk access, other tools test other
> aspects. I gave the hdparm numbers because everyone can reproduce them.
> For RAID0 with two disks you do see - using e.g. hdparm - the doubling
> of performance from two disks.
> If you take the time to read (or do) RAID benchmarks you'll discover
> that Linux software RAID1 is about as fast as a single disk (and RAID0
> with two disks is about twice the speed). It's as simple as that.
>
>
I beg to differ again since I did get combined throughput from a md
raid1 device. I would have saved them iostat output to disk if I had
known they would have some use. Anyway, I have got some numbers in my
other post but on an Ubuntu box.
>>> To get performance gain in RAID1 mode you need hardware RAID1.
>>>
>>>
>> Bollocks. The only area in which hardware raid has a significant
>> performance advantage over software raid is raid5/6 given sufficient
>> cache memory and processing power.
>>
>
> We were talking about RAID1; RAID5/6 is a different area. Linux software
> RAID1 is a safeguard against disk failure; it's not designed for speed
> increase. There is a number of things that could be improved in Linux
> software RAID; read performance of RAID1 is one of them - this _is_ why
> some hardware RAID1 adapters indeed are faster than software.
> Read http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing - since
> the 2.6.25 kernel a simple alternating read is implemented, but that
> does not take the access pattern into account.
>
I have not read that yet but that is odd since I have been blasted by
others before for doubting md raid1 doing multiple disk reads.
BTW, the Hardy box's kernel is 2.6.24-22-generic. I guess I need to try
to generate some from an Intrepid box and see if I get better numbers.
> So Linux software RAID1 is just mirroring - and it's good at that.
>
>
It has gotten good...no more having to sync from the beginning to end I
believe...just like some hardware raid cards.
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