> On 7 Jul 2016, at 00:22, Kai Krakow <hurikha...@gmail.com> wrote: > > Am Wed, 6 Jul 2016 13:20:15 +0100 > schrieb Tomasz Kusmierz <tom.kusmi...@gmail.com>: > >> When I think of it, I did move this folder first when filesystem was >> RAID 1 (or not even RAID at all) and then it was upgraded to RAID 1 >> then RAID 10. Was there a faulty balance around August 2014 ? Please >> remember that I’m using Ubuntu so it was probably kernel from Ubuntu >> 14.04 LTS >> >> Also, I would like to hear it from horses mouth: dos & donts for a >> long term storage where you moderately care about the data: RAID10 - >> flaky ? would RAID1 give similar performance ? > > The current implementation of RAID0 in btrfs is probably not very > optimized. RAID0 is a special case anyways: Stripes have a defined > width - I'm not sure what it is for btrfs, probably it's per chunk, so > it's 1GB, maybe it's 64k **. That means your data is usually not read > from multiple disks in parallel anyways as long as requests are below > stripe width (which is probably true for most access patterns except > copying files) - there's no immediate performance benefit. This holds > true for any RAID0 with read and write patterns below the stripe size. > Data is just more evenly distributed across devices and your > application will only benefit performance-wise if accesses spread > semi-random across the span of the whole file. And at least last time I > checked, it was stated that btrfs raid0 does not submit IOs in parallel > yet but first reads one stripe, then the next - so it doesn't submit > IOs to different devices in parallel. > > Getting to RAID1, btrfs is even less optimized: Stripe decision is based > on process pids instead of device load, read accesses won't distribute > evenly to different stripes per single process, it's only just reading > from the same single device - always. Write access isn't faster anyways: > Both stripes need to be written - writing RAID1 is single device > performance only. > > So I guess, at this stage there's no big difference between RAID1 and > RAID10 in btrfs (except maybe for large file copies), not for single > process access patterns and neither for multi process access patterns. > Btrfs can only benefit from RAID1 in multi process access patterns > currently, as can btrfs RAID0 by design for usual small random access > patterns (and maybe large sequential operations). But RAID1 with more > than two disks and multi process access patterns is more or less equal > to RAID10 because stripes are likely to be on different devices anyways. > > In conclusion: RAID1 is simpler than RAID10 and thus its less likely to > contain flaws or bugs. > > **: Please enlighten me, I couldn't find docs on this matter.
:O It’s an eye opener - I think that this should end up on btrfs WIKI … seriously ! Anyway my use case for this is “storage” therefore I predominantly copy large files. > -- > 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 -- 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
