On Wed, Apr 1, 2015 at 1:36 PM, Piergiorgio Sartor <piergiorgio.sartor@xxxxxxxx> wrote: > On Tue, Mar 31, 2015 at 08:47:04PM -0700, Dan Williams wrote: >> 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. > > There is one thing I do not really get. > > The target is to avoid the "write hole", which happens, > for example, when there is a sudden power failure. > > Now, how can be assured, in that case, that the "cache" > device is safe after the power is restored? You do sync write-ahead logging on the Flash cache. If it return successful, you do fire the writes to the RAID. If system crash/fails during the RAID writes (Write-hole), you just recover data by scanning write-ahead log in the flash cache and replay the logs into the RAID drives. > Doesn't this solution just shifts the problem from > the array to a different device (SSD, for example)? I don't see such a shift. Enterprise hardware RAIDs also used similar technique to fix the write-hole issue. -- 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
