Re: raid 0 read performance

[Mark Overmeer <solutions@overmeer.net>]

* Raz Ben-Jehuda(caro) (raziebe@gmail.com) [051229 10:10]:
> I have tested the overhead of linux raid0.
> I used two scsi atlas maxtor disks ( 147 MB) and combined them to single
> raid0 volume.
> The raid is striped in 256K stripes.

Are you sure you tested "linux" overhead?  Maybe you have just tested raid0
properties.

> I filled the raid0 up to the maximum with files over xfs file system.
> I've checked the peformance of reading 60 files like that:
> 
> while need to read
>  for every file
>      read 0.5 M from file
> 
> I got 50 MB/s .
> 
> Armed with this knowledge i went and did the same test over
> one disk and i got 32 MB/s .
> 
> Question:
>  why is this perfomance drop ?

Good performance for such small reads wrt the block-size...
Ok, simple calculation:
  seek time average for one disk is half a rotation
  for two disks   1 - (1-0.5)*(1-0.5) = 0.75 rotation

Without taking into account the time to do actual reads:
  0.5/0.75  * (2x32MB/s) = 43MB/s     (34% performance drop)

The only effect you see is that the probability that both disks are in
optimal position to read from them decreases.

Solution: take very large files wrt the stripe-size to get double
performance.  Or take files smaller than the stripe-size.

Of course, there can be other reasons which can reduce the performance
as well.  However, I achieve 200MB/s over 4 striped disk each capable
of 50MB/s for huge files and 64K stipes... Linux doesn't seem to be the
bottleneck in my setup.
-- 
Regards,
               MarkOv

------------------------------------------------------------------------
       Mark Overmeer MSc                                MARKOV Solutions
       Mark@Overmeer.net                          solutions@overmeer.net
http://Mark.Overmeer.net                   http://solutions.overmeer.net