Re: raid1 performance

[Neil Brown <nfbrown@novell.com>]

On Tue, 27 Jul 2010 16:10:36 +0000 (GMT)
Marco <jjletho67-diar@yahoo.it> wrote:

> 
> 
> 
> 
> >If you look at /proc/diskstats and particularly the 4th and 6th fields for
> >the device that you are interested in, and then take the differences for each
> >field between 'before' and 'after' running a test you will get
> > - the number of IO requests
> >  - the number of sectors
> >
> >that were serviced during that time.  Taking a ratio will get you the number
> >of sectors per IO.  Normally more is better.
> 
> Hi Neil,
> are you sure the right value are the 4th and the 6th fields ? I see strange 
> value in them (the 4th field is bigger then the 6th while i was expecting the 
> contrary)
> looking in the iostats.txt file (kernel documentation) i suspect the fields you 
> are interested are the first and the third.

No, the first field is the major device number, and the third is the device
name ....
so I guess we are taking the same ratio, but you start counting at a
different place to me.

> In this hypothesis (1st and 3rd fields) the ratio i obtain are:
> 
> sda 84.02 
> sdb 113,69
> md2 8,01
> 
> md2 and his meber have a very different ratio....

Strange isn't it.  And the ratios are the other-way-around to what I get.
I don't currently understand why ... but it might not be at all relevant to
the speed difference.

> 
> >It might be interesting to find out what the data offset is for your RAID1
> >(mdadm --examine will tell you if there is one), and compare the
> > request/sector numbers and see if they show anything.
> 
> this is the output of mdadm --examine /dev/sda3
> 
> /dev/sda3:
>           Magic : a92b4efc
>         Version : 0.90.00

OK, so there is no data offset with 0.90, so that rules out differing offsets
being an issue.

> 
> Do you have some hypothesis ?

No.  It might be worth exploring why the request sizes are different - I
don't know if it will lead anywhere useful though.

NeilBrown

> 
> thank you all
> 
> Marco
> 
> 
> 
>