* Raz Ben-Jehuda(caro) (raziebe@xxxxxxxxx) [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@xxxxxxxxxxxx solutions@xxxxxxxxxxxx
http://Mark.Overmeer.net http://solutions.overmeer.net
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