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SubjectRe: software raid does not do parallel reads under 2.4?
Neil Brown wrote:
>
> On Thursday August 23, pcg( Marc)@goof(A.).(Lehmann )com wrote:
> > It seems that the md driver in striped mode does not parallelize any reads
> > under 2.4. The scenario is that I have 3 disks on different controllers
> > (different pci slots), so there should not be any ide bus contention.
> >
> > When I read any single disk (e.g. using hdparm or dd), I get 32MB/s. When
> > I read two of them at the same time, I get about 28MB/s for each disk.
> >
> > Under linux-2.2 using md and striping I get about 40-50MB/s, whereas, under
> > 2.4, the same raid gives about 30MB/s.
> >
> > I then reformatted the raid to have 2MB chunksize. If I look at the disk
> > LED's while reading from them (e.g. dd if=/dev/md3 bs=1024x1024x8), I see
> > that each disk is read in turn, while the other two disks are idle.
> >
> > so it looks as if md under 2.4 only reads disks in turn, which makes
> > striping useless as a performance tool.
>
> For raid0, the md driver just redirects requests to the right drive.
> It doesn't explicitly serialise or parallelise anything. 2.4 works in
> exactly the same was as 2.2.
>
> With a 2MB chunksize, I would expect a linear read to touch just one
> drive at a time.
> With a 4K chunk size, I suspect that an linear read would read from
> all the drives in parallel.
> You say that you reformatted to 2MB chunksize. What did you reformat
> from?
>
> NeilBrown

I experienced some performance loss when moving from kernel 2.4.4 to
2.4.7-ac3 regarding ide harddisks. I am useing the hpt ataraid driver
which does pretty much the same thing as the md disk striping driver.
I/O speed of the raid volume is about as fast as accessing a single
drive.
There were a lot of ide reports some time ago. Maybe they where problems
with concurrent I/O operations...?

Wilfried
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