On Thu, Jan 26, 2012 at 10:25:33PM -0700, Andreas Dilger wrote: [snip] > Doesn't the kernel derive at least some idea of the speed of a device > due to the writeback changes that you made? It would be very useful > if we could derive at least some rough metric for the device performance > in the kernel and use that as input to the readahead window size as well. Yeah we now have bdi->write_bandwidth (exported as "BdiWriteBandwidth" in /debug/bdi/8:0/stats) for estimating the bdi write bandwidth. However the value is not reflecting the sequential throughput in some cases: 1) when doing random writes 2) when doing mixed reads+writes 3) when not enough IO have been issued 4) in the rare case, when writing to a small area repeatedly so that it's effectively writing to the internal disk buffer at high speed So there are still some challenges in getting a reliably usable runtime estimation. Thanks, Fengguang -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
