4/16/2023 0 Comments Block counter 1 megabyte dd![]() ![]() I know when I'm copying hard disks I get a faster rate by specifying bs=1M than by using bs=4k or the default. if you are actually copying blocks to another device you have pauses in the reading while the other device is accepting the data you want to write, when this happens you can hit rotational latency issues on the read device (if it's a hard disk) and so it's often significantly faster to read 1M chunks off the HDD as you come up against rotational latency less often that way. What you have done is only a read speed test. Having said all the above, now the question: can someone explain (a kernel hacker?) what are the major component/systems involved in such throughput, and if it really worth the effort in specifying a bs higher than the default? ![]() significant as well.Īlso, it was very clear that the CPU sys-time was inversely proportional to the bs value (but this sounds reasonable, as the lower the bs, the higher the number of sys-calls generated by dd). Surely lower than the 95.3 MB got with the default bs=512 but. A not so distant values wrt to the maximum 14.2/14.3 that I got from bs=5 and above ![]() MMC: with a bs=4 (yes! 4 bytes), I reached a throughput of 12MB/s. In the following table, I've reported my findings, reading 1GB of data with different values of "bs" ( you can find the raw numbers at the end of this message): avoiding some basic issues of HDD-caching, at least when involving the HDD.sending the output to /dev/null to avoid issues related to "writing speed".In order to lower external influences, I decided to read: the perfect size will depend on your system bus, hard drive controller, the particular drive itself, and the drivers for each of those." (chris-s)Īs my feeling was a bit different ( BTW: I tought that the time needed to deeply-tune the bs parameter was much higher than the gain received, in terms of time-saved, and that the default was reasonable), today I just went through some quick-and-dirty benchmarks. ![]() the optimum block size is hardware dependent." (iain) or ". Every now and then, I'm told that to increase the speed of a "dd" I should carefully choose a proper "block size".Įven here, on ServerFault, someone else wrote that ". ![]()
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