On Wed, Mar 9, 2016 at 2:36 PM, Roman Mamedov <r...@romanrm.net> wrote: > On Wed, 9 Mar 2016 15:25:19 -0500 > Nicholas D Steeves <nstee...@gmail.com> 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. > > No RAID1 implementation reads from disks in a round-robin fashion, as that > would give terrible performance giving disks a constant seek load instead of > the normal linear read scenario. > > As for reading at the same time, there's no reason to do that either, since > the data integrity is protected by checksums, and "the other" disk for a > particular data piece is being consulted only in case the checksum did not > match (or when you execute a 'scrub'). > > It's a known limitation that the disks are in effect "pinned" to running > processes, based on their process ID. One process reads from the same disk, > from the point it started and until it terminates. Other processes by luck may > read from a different disk, thus achieving load balancing. Or they may not, > and you will have contention with the other disk idling. This is unlike MD > RAID1, which knows to distribute read load dynamically to the least-utilized > array members.
This is a better qualification than my answer. > > Now if you want to do some more performance evaluation, check with your dstat > if both disks happen to *write* data in parallel, when you write to the array, > as ideally they should. Last I checked they mostly didn't, and this almost > halved write performance on a Btrfs RAID1 compared to a single disk. I've found it to be about the same or slightly less than single disk. But most of my writes to raid1 are btrfs receive. -- Chris Murphy -- 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
