Hi, I've been experimenting using bcache and MD RAID on Linux 4.14.91. I have a 12-disk RAID6 MD array as the backing device, and a decent NVMe SSD as the caching device. I'm testing using write-back mode. I've been able to tune the sequential_cutoff so when issuing full stripe writes to the bcache device, these bypass hitting the cache device and go right into the MD RAID6 array, which seems to be working nicely. In the next experiment, when performing more random / sequential (mixed) writes, the cache device does a nice job of keeping up performance. However, when watching the data get flushed from the cache device to the backing device (the MD RAID6 volume), it doesn't seem the data is being written out as mostly full stripe writes. I get a lot of RMW's on the drives, so I don't believe I'm seeing these full stripe writes. I was sort of hoping/expecting bcache to do some re-ordering with this... there seem to be some knobs in bcache where it detects the full stripe size, and it knows partial stripe writes are expensive. So I guess my question is if it's known that the data is not re-ordered using full stripe geometry in bcache, or perhaps this is just a tunable that I'm not seeing? It seems bcache has access to this data, but maybe this is a future item where it could be implemented? The problem of course comes from the the sub-par performance when data is flushed from the cache device to the backing device... lots of read-modify-writes result in very poor write performance. If the I/O was pushed to the backing device as full stripe I/O's (or at least mostly) I'd expect to see better performance when flushing the cache. Thanks for your time. --Marc
