Hello, I have a performance issue with «btrfs replace» with raid5 and a _missing_ device. My btrfs rely on 6x4TB HDD and the operating system is an Archlinux.
In a nutshell, I will need 23 to 46 days to replace on missing disk. # btrfs fi sh /home Label: 'raptor.home' uuid: 8739c8b2-110b-44ac-8b4d-285ad06ee446 Total devices 7 FS bytes used 14.60TiB devid 0 size 3.64TiB used 2.80TiB path /dev/sdf devid 3 size 3.64TiB used 2.97TiB path /dev/sdh devid 5 size 3.64TiB used 2.97TiB path /dev/sdc devid 6 size 3.64TiB used 2.97TiB path /dev/sdd devid 7 size 3.64TiB used 2.97TiB path /dev/sde devid 8 size 3.64TiB used 2.97TiB path /dev/sdg *** Some devices missing At a disk full speed (100 MB/s) replacing the missing disk (4 TB) should take around 8 hours. With the same disk model and same HBA card in another computer with a mdadm/raid5, I could verify this duration could be reach. I also tested a «btrfs replace» without a missing disk and the speed was not so bad. Somewhere around half disk speed (50-60MB/s). Performances are under mdadm. But, in my case, the drive is pass away, I can't use it as source of the replace, so I have a replace speed of 1-2MB/s ! Which mean between 23-46 days with bad usage performance and security risk. I tried to upgrade the kernel to the latest (4.7-rc6) but it's not better in performance. I got some crash during replace with 4.6.0 which vanish with the last rc. # iostat -md Linux 4.7.0-rc6-seblu (raptor.seblu.net) 14/07/2016 _x86_64_ (4 CPU) Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn sdc 356,75 22,51 0,14 9132054 58427 sdd 356,27 22,51 0,14 9131612 57094 sde 361,53 22,52 0,14 9132207 57245 sdf 362,78 0,00 1,81 4 735786 sdg 357,82 22,51 0,14 9131763 58323 sdh 325,25 22,52 0,14 9132715 58355 So I have a really poor performance in rebuilding a raid5 mostly when the replaced device is missing. Is there a parameter to tweak of something I can do to improve the replace ? Regards, -- Sébastien "Seblu" Luttringer https://seblu.net | Twitter: @seblu42 GPG: 0x2072D77A
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