Dne 5.4.2011 18:25, Ray Morris napsal(a): >> Isn't this already done in a better way by 'dd' using 'zero' copy >> mechanism ?? (assuming it is already using either 'mmap()' -> >> 'write()' or 'splice()' as your proposed piped copy is doing a lot >> of pointless memory copies and cache trashing. > > No, with directio there isn't "a lot of pointless memory copies > and cache trashing". There are in fact fewer mempoy copies than > there would be with a simple: > dd if=thing of=thing > > See: > http://www.ukuug.org/events/linux2001/papers/html/AArcangeli-o_direct.html > Seem like 10 years old paper for 2.4.6 kernel - so I do not think it still applies to upstream 2.6.38 - thought I've not made any recent measuring myself. > The proof of the pudding is in the eating, and in fact that > recipe is extremely fast, for this application (sequential > copy of GBs from one device to another). I spent a couple > of hours testing before spending hundreds of hours copying. > My bottom line was speed, so I didn't even look at memory > usage or CPU. I would't mind using an extra 64 MB in order > to get my copying done 30 hours faster. The kernel should be able to use as much free memory as you have - and when it's properly configured - disk flushing running in separate thread should ensure maximal throughput. If you are experiencing significant performance degradation with usage of single 'dd' with two different driver compared with 2 piped 'dd' - you should probably make a report on lkml list - There is something broken if you gain 'hours' like you said by piping 2 dd commands. (I don't have fast enough arrays myself for such playing) Zdenek _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
