Messages in this thread | | | Date | Mon, 7 Jan 2008 13:49:52 +0100 | From | Jörn Engel <> | Subject | Re: [noob q. on block layer] block IO read-ahead during sequential *write*? |
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On Mon, 7 January 2008 13:25:09 +0100, Frantisek Rysanek wrote: > > let me start with a simple example. The following commands: > > cp /dev/zero /dev/hda > dd if=/dev/zero of=/dev/hda [bs=512] > > both have one common side-effect: apart from the disk being properly > overwritten with zeroes, the kernel seems to keep reading sectors > ahead of the current seek position of the sequential write.
Block devices are cached in the page cache. If you write less than a full page, any remainder has to be read from the device.
If you retry the dd with bs=4096 (or whatever your architecture's page size happens to be), does this still occur?
Jörn
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