On 2019/6/29 5:56 上午, Don Doerner wrote: > Hello, > I'm also interested in using bcache to facilitate stripe re-ass'y for the backing device. I've done some experiments that dovetail with some of the traffic on this mailing list. Specifically, in this message (https://www.spinics.net/lists/linux-bcache/msg07590.html), Eric suggested "...turning up /sys/block/bcache0/bcache/writeback_percent..." to increase the contiguous data in the cache. > My RAID-6 has a stripe size of 2.5MiB, and its bcache'ed with a few hundred GB of NVMe storage. Here's my experiment: > * I made the cache a write back cache: echo writeback > /sys/block/bcache0/bcache/cache_mode > * I plugged the cache: echo 0 > /sys/block/bcache0/bcache/writeback_running > * I use a pathological I/O pattern, generated with 'fio': fio --bs=128K --direct=1 --rw=randwrite --ioengine=libaio --iodepth=1 --numjobs=1 --size=40G --name=/dev/bcache0. I let it run to completion, at which point I believe I should have 40 GiB of sequential dirty data in cache, but not put there sequentially. In essence, I should have ~16K complete stripes sitting in the cache, waiting to be written. > * I set stuff up to go like a bat: echo 0 > /sys/block/bcache0/bcache/writeback_percent; echo 0 > /sys/block/bcache0/bcache/writeback_delay; echo 2097152 > /sys/block/bcache0/bcache/writeback_rate > * And I unplugged the cache: echo 1 > /sys/block/bcache0/bcache/writeback_running > I then watched 'iostat', and saw that there were lots of read operations (statistically, after merging, about 1 read for every 7 writes) - more than I had expected... that's enough that I concluded it wasn't building full stripes. It kinda looks like it's playing back a journal sorted in time then LBA, or something like that... > Any suggestions for improving (reducing) the ratio of reads to writes will be gratefully accepted! Hi Don, If the backing device has expensive stripe cost, the upper layer should issue I/Os with stripe size alignment, otherwise bcache cannot to too much to make the I/O to be stripe optimized. And you are right that bcache does not writeback in restrict LBA order, this is because the internal btree is trend to be appended only. The LBA ordering writeback happens in a reasonable small range, not in whole cached data, see commit 6e6ccc67b9c7 ("bcache: writeback: properly order backing device IO"). And I agree with you again that "improving (reducing) the ratio of reads to writes will be gratefully accepted". Indeed not only reducing reads to writes ratio, but also increase the reads to writes throughput. This is something I want to improve, after I understand why the problem exists in bcache writeback code ... Thanks. -- Coly Li
