On Mon, Mar 30, 2015 at 3:25 PM, Shaohua Li <shli@xxxxxx> wrote: > This is my attempt to fix raid5/6 write hole issue, it's not for merge > yet, I post it out for comments. Any comments and suggestions are > welcome! > > Thanks, > Shaohua > > We expect a completed raid5/6 stack with reliability and high > performance. Currently raid5/6 has 2 issues: > > 1. read-modify-write for small size IO. To fix this issue, a cache layer > above raid5/6 can be used to aggregate write to full stripe write. > 2. write hole issue. A write log below raid5/6 can fix the issue. > > We plan to use a SSD to fix the two issues. Here we just fix the write > hole issue. > > 1. We don't try to fix the issues together. A cache layer will do write > acceleration. A log layer will fix write hole. The seperation will > simplify things a lot. > > 2. Current assumption is flashcache/bcache will be used as the cache > layer. If they don't work well, we can fix them or add a simple cache > layer for raid write aggregation later. We also assume cache layer will > absorb write, so log doesn't worry about write latency. It seems neither bcache nor dm-cache are tackling the write-buffering problem head on... they still seem to be concerned with some amount of read caching which I can see as useful for file servers and workstations, but not necessarily scale out storage. I'll try to set aside time to take a look at the patch this week. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
