Re: RAID-1 to RAID-5 online migration?
On Mon, 20 Sep 2004 21:37, Josh Bonnett <nodus@brave.cs.uml.edu> wrote:
> >Do you have benchmark results to support this assertion? Last time I
> > tested the performance of software RAID-1 on Linux I was unable to get
> > anywhere near 2x disk speed for writing.
>
> Not to be a stickler but i hope you mean reading
Correct.
> > I did tests by reading two files that were 1G in
> >size and the operation took considerably longer than reading a single 1G
> > file from a non-RAID system. If RAID-1 was delivering twice the read
> > throughput then I should be able to read two 1G files concurrently from a
> > RAID-1 in the same time as would be taken to read a single 1G file from a
> > single disk.
>
> also i think that the original poster was assuming that the raid 1
> driver would read in stripes as you would a raid 0, its not required to
> implement raid 1 as far as i know, but it might be a nice thing to add
> (not sure if it fits nicely in the Linux software raid drivers). You
> might have to trade some memory usage and cpu to make sure the blocks
> were put back together again(a fifo buffer 2 stripe sizes big would
> probably be all it would need) so whether it helps would all come down
> to where the bottleneck is.
I believe that the Linux software RAID-1 code already spreads the read load
between the disks. The problem is that the algorithm chosen is not much good
(or at least was not much good last time I tested it).
If you had RAID-1 running over flash disks, NVRAM devices or other media that
does not have a seek overhead then I expect that read performance would
double whenever you have at least two processes reading from the same RAID-1
device. But when seeks matter the algorithm loses.
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