Am Mon, 6 Feb 2017 08:19:37 -0500 schrieb "Austin S. Hemmelgarn" <ahferro...@gmail.com>:
> > MDRAID uses stripe selection based on latency and other measurements > > (like head position). It would be nice if btrfs implemented similar > > functionality. This would also be helpful for selecting a disk if > > there're more disks than stripesets (for example, I have 3 disks in > > my btrfs array). This could write new blocks to the most idle disk > > always. I think this wasn't covered by the above mentioned patch. > > Currently, selection is based only on the disk with most free > > space. > You're confusing read selection and write selection. MDADM and > DM-RAID both use a load-balancing read selection algorithm that takes > latency and other factors into account. However, they use a > round-robin write selection algorithm that only cares about the > position of the block in the virtual device modulo the number of > physical devices. Thanks for clearing that point. > As an example, say you have a 3 disk RAID10 array set up using MDADM > (this is functionally the same as a 3-disk raid1 mode BTRFS > filesystem). Every third block starting from block 0 will be on disks > 1 and 2, every third block starting from block 1 will be on disks 3 > and 1, and every third block starting from block 2 will be on disks 2 > and 3. No latency measurements are taken, literally nothing is > factored in except the block's position in the virtual device. I didn't know MDADM can use RAID10 on odd amounts of disks... Nice. I'll keep that in mind. :-) -- Regards, Kai Replies to list-only preferred. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
