On Wed, Mar 9, 2016 at 1:25 PM, Nicholas D Steeves <nsteeves@xxxxxxxxx> wrote: > grr. Gmail is terrible :-/ > > I understood that a btrfs RAID1 would at best grab one block from sdb > and then one block from sdd in round-robin fashion, or at worse grab > one chunk from sdb and then one chunk from sdd. Alternatively I > thought that it might read from both simultaneously, to make sure that > all data matches, while at the same time providing single-disk > performance. None of these was the case. Running a single > IO-intensive process reads from a single drive. > > Did I misunderstand the documentation and is this normal, or is this a bug? > Nicholas > > On 9 March 2016 at 15:21, Nicholas D Steeves <nsteeves@xxxxxxxxx> wrote: >> Hello everyone, >> >> I've run into an expected behaviour for a my two disk RAID1. I mount >> with UUIDs, because sometimes my USB disk gets /dev/sdc instead of >> /dev/sdd. The two elements of my RAID1 are currently sdb and sdd. >> >> dstat -tdD total,sdb,sdc,sdd >> >> It seems that per process, reads come from either sdb or sdd. This >> surprises me, because I understood that a btrfs RAID1 It's normal and recognized to be sub-optimal. So it's an optimization opportunity. :-) I see parallelization of reads and writes to data single profile multiple devices as useful also, similar to XFS allocation group parallelization. Those AGs are spread across multiple devices in md/lvm linear layouts, so if you have processes that read/write to multiple AGs at a time, those I/Os happen at the same time when on separate devices. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
