doesm mdadm try to use fastest HDD ?

doesm mdadm try to use fastest HDD ?

[Janek Kozicki]

Hello,

My three HHDs have following speeds:

  hda - speed 70 MB/sec
  hdc - speed 27 MB/sec
  sda - speed 60 MB/sec

They create a raid1 /dev/md0 and raid5 /dev/md1 arrays. I wanted to
ask if mdadm is trying to pick the fastest HDD during operation?

Maybe I can "tell" which HDD is preferred?

This came to my mind when I saw this:

  # mdadm --query --detail /dev/md1 | grep Prefer
 
  Preferred Minor : 1

And also in the manual:

  -W, --write-mostly [...] "can be useful if mirroring over a slow link."


many thanks for all your help!
-- 
Janek Kozicki                                                         |

Re: doesm mdadm try to use fastest HDD ?

[Bill Davidsen]

Janek Kozicki wrote:
> Hello,
>
> My three HHDs have following speeds:
>
>   hda - speed 70 MB/sec
>   hdc - speed 27 MB/sec
>   sda - speed 60 MB/sec
>
> They create a raid1 /dev/md0 and raid5 /dev/md1 arrays. I wanted to
> ask if mdadm is trying to pick the fastest HDD during operation?
>
> Maybe I can "tell" which HDD is preferred?
>   

If you are doing raid-1 between hdc and some faster drive, you could try 
using write-mostly and see go that works for you. For raid-5, it's 
faster to read the data off the slow drive than reconstruct it with 
multiple reads to multiple othjer faster drives.
> This came to my mind when I saw this:
>
>   # mdadm --query --detail /dev/md1 | grep Prefer
>  
>   Preferred Minor : 1
>
> And also in the manual:
>
>   -W, --write-mostly [...] "can be useful if mirroring over a slow link."
>
>
> many thanks for all your help!
>   
I have two thoughts on this:
1 - if performance is critical, replace the slow drive
2 - for most things you do, I would expect seek to be more important 
than transfer rate

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

Re: doesm mdadm try to use fastest HDD ?

[Goswin von Brederlow]

Bill Davidsen <davidsen@tmr.com> writes:

> Janek Kozicki wrote:
>> Hello,
>>
>> My three HHDs have following speeds:
>>
>>   hda - speed 70 MB/sec
>>   hdc - speed 27 MB/sec
>>   sda - speed 60 MB/sec
>>
>> They create a raid1 /dev/md0 and raid5 /dev/md1 arrays. I wanted to
>> ask if mdadm is trying to pick the fastest HDD during operation?
>>
>> Maybe I can "tell" which HDD is preferred?
>>
>
> If you are doing raid-1 between hdc and some faster drive, you could
> try using write-mostly and see go that works for you. For raid-5, it's
> faster to read the data off the slow drive than reconstruct it with
> multiple reads to multiple othjer faster drives.

Won't it naturally balance reads according to the disk speeds if there
are enough streams reading? I know raid1 prefers a disk that is
already near the location it wants. But won't it pick the first idle
one if there is no disk near the location? And the faster disk will be
idle more often.

MfG
        Goswin