Messages in this thread |  | | | Date | Mon, 21 Feb 2005 13:31:39 -0500 | | From | Jeff Garzik <> | | Subject | Re: Merging fails reading /dev/uba1 |
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On Mon, Feb 21, 2005 at 10:24:31AM -0800, Pete Zaitcev wrote: > On Mon, 21 Feb 2005 08:51:32 +0100, Jens Axboe <axboe@suse.de> wrote: > > > > [root@lembas ~]# time dd if=/dev/uba of=/dev/null bs=10k count=10240 > > > real 0m22.731s > > > > [root@lembas ~]# time dd if=/dev/uba1 of=/dev/null bs=10k count=10240 > > > real 1m42.622s > > > > So, reading from a partition of the same device is 5 times slower than > > > reading from the device itself. The question is, why? > > > I can't explain why the replugging slows it down, maybe you were lucky > > to get contigious pages in the first case? As far as I can see, ub > > effectively disables merging by setting max hw/phys segment limit of 1. > > If you mean physical replugging, it has nothing to do with the issue. > I only mentioned it to show that old pages were purged. > > Contiguous pages have nothing to do with it either. I forgot to mention > that in the first case (whole device), all reads are done with length of > 4KB, while in the second case (partition), all reads are 512 bytes long. > > Basically, the key is reading from a partition or not. It causes the > sub-page sized merging to fail.
Does setting the blkdev's block size change things?
Jeff
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