Re: blocksize when using dd to copy disks? bigger = better?
On Tue, Sep 23, 2008 at 11:04:13AM -0400, Joachim Rosenfeld wrote: When mirroring a disk with dd, I notice that a blocksize of 512 runs awfully slow, but with bs=1MB (2^10bytes), it runs fairly quickly. Can someone explain the implications of this? Did all the data not copy properly with the larger blocksize? If you are on a beach moving sand and you pick up one grain at a time and move it, it will take a very long time because the overhead of moving yourself is much higher than the amount of sand moved. If you use the largest bucket or scoop that you can handle, then it goes much faster because the same body motions result in much more being moved.Moving data has a similar dynamic. All the data will get copied properly in either case unless there is some big error. A big error is probably more likely to happen with the smaller block size just because there are more operations in which an error can occur. jerry thanks Joe ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED] ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
Re: blocksize when using dd to copy disks? bigger = better?
On Tue, 23 Sep 2008 11:37:00 -0400 Jerry McAllister [EMAIL PROTECTED] wrote: On Tue, Sep 23, 2008 at 11:04:13AM -0400, Joachim Rosenfeld wrote: When mirroring a disk with dd, I notice that a blocksize of 512 runs awfully slow, but with bs=1MB (2^10bytes), it runs fairly quickly. Can someone explain the implications of this? Did all the data not copy properly with the larger blocksize? If you are on a beach moving sand and you pick up one grain at a time and move it, it will take a very long time because the overhead of moving yourself is much higher than the amount of sand moved. If you use the largest bucket or scoop that you can handle, then it goes much faster because the same body motions result in much more being moved.Moving data has a similar dynamic. I tried playing around with this once, and I found that the speed rose rapidly up to a certain blocksize, then levelled-out for a decade or so and then dropped to half of the peak speed. IIRC in that particular case the optimum range was something like 20k-200k. I presume what happens is that you can make the blocksize too big for the other buffering, and end-up alternating reads and writes rather than doing them in parallel. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to [EMAIL PROTECTED]
