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. 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). > 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. >> [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. :-) >From a performance standpoint, absolutely not. Though I wouldn't be surprised if there are a few parity RAID implementations out there that do always write a full stripe for other reasons, such as catching media defects as early as possible, i.e. those occasions where bits in a sector may read just fine but can't be re-magnetized. I'm not championing such an idea, merely stating that others may use this method for this or other reasons. [1] On 6/25/2012 9:30 PM, Dave Chinner wrote: > You can't, simple as that. The maximum supported is 256k. As it is, > a default chunk size of 512k is probably harmful to most workloads - > large chunk sizes mean that just about every write will trigger a > RMW cycle in the RAID because it is pretty much impossible to issue > full stripe writes. Writeback doesn't do any alignment of IO (the > generic page cache writeback path is the problem here), so we will > lamost always be doing unaligned IO to the RAID, and there will be > little opportunity for sequential IOs to merge and form full stripe > writes (24 disks @ 512k each on RAID6 is a 11MB full stripe write). > > 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. > 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.... -- Stan -- 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
