Am 04.09.2017 um 12:53 schrieb Henk Slager: > On Sun, Sep 3, 2017 at 8:32 PM, Stefan Priebe - Profihost AG > <s.priebe@xxxxxxxxxxxx> wrote: >> Hello, >> >> i'm trying to speed up big btrfs volumes. >> >> Some facts: >> - Kernel will be 4.13-rc7 >> - needed volume size is 60TB >> >> Currently without any ssds i get the best speed with: >> - 4x HW Raid 5 with 1GB controller memory of 4TB 3,5" devices >> >> and using btrfs as raid 0 for data and metadata on top of those 4 raid 5. >> >> I can live with a data loss every now and and than ;-) so a raid 0 on >> top of the 4x radi5 is acceptable for me. >> >> Currently the write speed is not as good as i would like - especially >> for random 8k-16k I/O. >> >> My current idea is to use a pcie flash card with bcache on top of each >> raid 5. > > If it can speed up depends quite a lot on what the use-case is, for > some not-so-much-parallel-access it might work. So this 60TB is then > 20 4TB disks or so and the 4x 1GB cache is simply not very helpful I > think. The working set doesn't fit in it I guess. If there is mostly > single or a few users of the fs, a single pcie based bcacheing 4 > devices can work, but for SATA SSD, I would use 1 SSD per HWraid5. Yes that's roughly my idea as well and yes the workload is 4 users max writing data. 50% sequential, 50% random. > Then roughly make sure the complete set of metadata blocks fits in the > cache. For an fs of this size let's say/estimate 150G. Then maybe same > of double for data, so an SSD of 500G would be a first try. I would use 1TB devices for each Raid or a 4TB PCIe card. > You give the impression that reliability for this fs is not the > highest prio, so if you go full risk, then put bcache in write-back > mode, then you will have your desired random 8k-16k I/O speedup after > the cache is warmed up. But any SW or HW failure wil result in total > fs loss normally if SSD and HDD get out of sync somehow. Bcache > write-through might also be acceptable, you will need extensive > monitoring and tuning of all (bcache) parameters etc to be sure of the > right choice of size and setup etc. Yes i wanted to use the write back mode. Has anybody already made some test or experience with a setup like this? Greets, Stefan -- 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
