Messages in this thread |  | | Date | Fri, 24 Aug 2001 19:01:40 +0200 | From | Wilfried Weissmann <> | Subject | Re: software raid does not do parallel reads under 2.4? |
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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...?
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