On Sun, Aug 19, 2012 at 11:44:25PM -0500, Stan Hoeppner wrote: > I'm copying Dave C. as he apparently misunderstood the behavior of > md/RAID6 as well. My statement was based largely on Dave's information. > See [1] below. Not sure what I'm supposed to have misunderstood... > On 8/19/2012 7:01 PM, NeilBrown wrote: > > On Sun, 19 Aug 2012 18:34:28 -0500 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> > > wrote: > > > Since we are trying to set the record straight.... > > Thank you for finally jumping in Neil--had hoped to see your > authoritative information sooner. > > > md/RAID6 must read all data devices (i.e. not parity devices) which it is not > > going to write to, in an RWM cycle (which the code actually calls RCW - > > reconstruct-write). That's a RMW cycle from an IO point of view. i.e. sycnhronous read must take place before the data can be modified and written... > > md/RAID5 uses an alternate mechanism when the number of data blocks that need > > to be written is less than half the number of data blocks in a stripe. In > > this alternate mechansim (which the code calls RMW - read-modify-write), > > md/RAID5 reads all the blocks that it is about to write to, plus the parity > > block. It then computes the new parity and writes it out along with the new > > data. And by the same definition, that's also a RMW cycle. > >> [1}The only thing that's not clear at this point is if md/RAID6 also > >> always writes back all chunks during RMW, or only the chunk that has > >> changed. > > > Do you seriously imagine anyone would write code to write out data which it > > is known has not changed? Sad. :-) Two words: media scrubbing. > On 6/25/2012 9:30 PM, Dave Chinner wrote: > > IOWs, every time you do a small isolated write, the MD RAID volume > > will do a RMW cycle, reading 11MB and writing 12MB of data to disk. Oh, you're probably complaining about that write number. All I was trying to do was demonstrate what a worst case RMW cycle looks like. So by the above, that occurs when you have a same isolated write to each chunk of the stripe. A single write is read 11MB, write 1.5MB (data + 2 parity). It doesn't really change the IO latency or load, though, you've still got the same read-all, modify, write-multiple IO pattern.... > > Given that most workloads are not doing lots and lots of large > > sequential writes this is, IMO, a pretty bad default given typical > > RAID5/6 volume configurations we see.... Either way, the point I was making in the original post stands - RAID6 sucks balls for most workloads as they only do small writes in comparison to the stripe width of the volume.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
