Re: The huge different performance of sequential read between RAID0 and RAID5

[Yuehai Xu <yuehaixu@gmail.com>]

2010/1/28 Gabor Gombas <gombasg@sztaki.hu>:
> On Thu, Jan 28, 2010 at 09:31:23AM -0500, Yuehai Xu wrote:
>
>> >> md0 : active raid5 sdh1[7] sdg1[5] sdf1[4] sde1[3] sdd1[2] sdc1[1] sdb1[0]
>> >>       631353600 blocks level 5, 64k chunk, algorithm 2 [7/6] [UUUUUU_]
> [...]
>
>> I don't think any of my drive fail because there is no "F" in my
>> /proc/mdstat output
>
> It's not failed, it's simply missing. Either it was unavailable when the
> array was assembled, or you've explicitely created/assembled the array
> with a missing drive.

I noticed that, thanks! Is it usual that at the beginning of each
setup, there is one missing drive?

>
>> How do you know my RAID5 array has one drive missing?
>
> Look at the above output: there are just 6 of the 7 drives available,
> and the underscore also means a missing drive.
>
>> I tried to setup RAID5 with 5 disks, 3 disks, after each setup,
>> recovery has always been done.
>
> Of course.
>
>> However, if I format my md0 with such command:
>> mkfs.ext3 -b 4096 -E stride=16 -E stripe-width=*** /dev/XXXX, the
>> performance for RAID5 becomes usual, at about 200~300M/s.
>
> I suppose in that case you had all the disks present in the array.

Yes, I did my test after the recovery, in that case, does the "missing
drive" hurt the performance?

Thanks!

Yuehai
>
> Gabor
>
> --
>     ---------------------------------------------------------
>     MTA SZTAKI Computer and Automation Research Institute
>                Hungarian Academy of Sciences
>     ---------------------------------------------------------
>
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