On Tue, Sep 14, 2010 at 05:09:31AM +1000, dave b wrote: > On 14 September 2010 01:21, Johannes Stezenbach <js@xxxxxxxxx> wrote: > > On Fri, Sep 10, 2010 at 06:02:48PM +0200, Johannes Stezenbach wrote: > >> > >> Linear read heuristic might be a good guess, but it would > >> be nice to hear a comment from a vm/fs expert which > >> confirms this works as intended. > > > > Anyway I found lmdd (from lmbench) can do random reads, > > and indeed causes the data to enter the block (page?) cache, > > replacing the previous data. > > I am no expert, but what did you think would happen if you did dd > twice from /dev/zero? > but... Honestly what do you think will be cached? It's not from /dev/zero, it is from file to /dev/null. It all started with me wanting to compare disk read bandwidth vs. read bandwidth of my root partition via dm-crypt + LVM, and then wondering why dd from raw disk seemed to be cached while dd from crypted root partition didn't. Johannes -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>