I observed that on an otherwise idle cluster, scrubbing cannot fully utilise the speed of my HDDs.
`iostat` shows only 8-10 MB/s per disk, instead of the ~100 MB/s most HDDs can easily deliver.
Changing scrubbing settings does not help (see below).
Environment:
* 6 active+clean+scrubbing+deep
* Ceph version 16.2.7.
* BlueStore
* My cluster has many objects small objects ("402.32M objects, 38 TiB" from "ceph status") due to small files (4 - 32 KiB) on CephFS.
`iostat -x 5` with default settings:
Device r/s rkB/s rrqm/s %rrqm r_await rareq-sz w/s wkB/s wrqm/s %wrqm w_await wareq-sz d/s dkB/s drqm/s %drqm d_await dareq-sz f/s f_await aqu-sz %util
dm-0 198.60 6878.40 0.00 0.00 12.78 34.63 51.80 2612.80 0.00 0.00 14.82 50.44 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.30 91.24
dm-1 0.80 3.20 0.00 0.00 11.50 4.00 52.60 2582.40 0.00 0.00 13.69 49.10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.73 3.78
dm-10 11.20 71.20 0.00 0.00 0.09 6.36 145.80 583.20 0.00 0.00 0.14 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 2.62
dm-11 192.60 6737.60 0.00 0.00 10.74 34.98 34.80 1684.80 0.00 0.00 11.47 48.41 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.47 91.40
dm-12 245.40 10194.40 0.00 0.00 9.43 41.54 21.20 575.20 0.00 0.00 3.94 27.13 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.40 87.92
dm-13 30.80 1772.80 0.00 0.00 11.61 57.56 78.80 4507.20 0.00 0.00 19.78 57.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.92 9.54
dm-14 3.20 24.80 0.00 0.00 0.12 7.75 125.20 500.80 0.00 0.00 0.12 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.18
dm-15 2.80 19.20 0.00 0.00 0.14 6.86 105.40 421.60 0.00 0.00 0.05 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 1.76
dm-16 0.80 6.40 0.00 0.00 0.00 8.00 111.00 444.00 0.00 0.00 0.10 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 1.82
dm-17 10.80 76.80 0.00 0.00 0.09 7.11 151.40 605.60 0.00 0.00 0.08 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.92
dm-18 10.20 67.20 0.00 0.00 0.08 6.59 115.60 462.40 0.00 0.00 0.04 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.16
dm-19 10.20 56.80 0.00 0.00 0.10 5.57 109.00 436.00 0.00 0.00 0.07 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.34
dm-2 4.80 435.20 0.00 0.00 0.12 90.67 751.80 6292.80 0.00 0.00 0.07 8.37 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 16.14
dm-20 0.40 2.40 0.00 0.00 0.00 6.00 265.00 2459.20 0.00 0.00 0.10 9.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 5.36
dm-21 191.00 6105.60 0.00 0.00 6.34 31.97 67.80 3748.00 0.00 0.00 19.56 55.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.54 42.22
dm-3 1.00 8.80 0.00 0.00 0.00 8.80 91.00 364.00 0.00 0.00 0.04 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.54
dm-4 167.60 4973.60 0.00 0.00 10.15 29.68 49.20 2511.20 0.00 0.00 11.39 51.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.26 89.18
dm-5 11.20 73.60 0.00 0.00 0.12 6.57 124.40 497.60 0.00 0.00 0.07 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.16
dm-6 27.20 1644.80 0.00 0.00 12.22 60.47 57.20 3316.80 0.00 0.00 15.93 57.99 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.24 6.78
dm-7 217.40 8032.80 0.00 0.00 12.04 36.95 64.40 3654.40 0.00 0.00 23.69 56.75 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4.14 97.28
dm-8 10.80 70.40 0.00 0.00 0.15 6.52 111.80 447.20 0.00 0.00 0.04 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.32
dm-9 1.60 8.00 0.00 0.00 13.25 5.00 46.60 2563.20 0.00 0.00 9.08 55.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.44 3.70
md127 1.60 107.20 0.00 0.00 0.00 67.00 142.00 1856.00 0.00 0.00 0.05 13.07 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.80
nvme0n1 42.00 772.00 0.00 0.00 0.10 18.38 1001.00 10503.50 451.20 31.07 0.03 10.49 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 25.20
nvme1n1 37.20 248.00 0.00 0.00 0.09 6.67 609.80 6723.50 369.00 37.70 0.04 11.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.03 15.80
sda 0.80 3.20 0.00 0.00 11.50 4.00 33.60 2582.40 19.00 36.12 8.43 76.86 0.00 0.00 0.00 0.00 0.00 0.00 3.20 0.25 0.29 3.78
sdb 117.80 10195.20 127.80 52.04 11.53 86.55 18.80 575.20 2.40 11.32 3.93 30.60 0.00 0.00 0.00 0.00 0.00 0.00 2.80 3.93 1.44 87.90
sdc 4.60 1644.80 22.60 83.09 11.74 357.57 23.60 3316.80 33.60 58.74 8.53 140.54 0.00 0.00 0.00 0.00 0.00 0.00 1.80 4.56 0.26 6.74
sdd 109.40 4975.20 58.40 34.80 12.19 45.48 22.00 2511.20 27.20 55.28 5.75 114.15 0.00 0.00 0.00 0.00 0.00 0.00 2.80 6.07 1.48 89.16
sde 115.40 6563.20 77.00 40.02 12.21 56.87 26.20 1684.80 8.60 24.71 7.82 64.31 0.00 0.00 0.00 0.00 0.00 0.00 1.80 6.78 1.63 91.36
sdf 6.40 1772.80 24.40 79.22 14.84 277.00 39.20 4507.20 39.60 50.25 13.93 114.98 0.00 0.00 0.00 0.00 0.00 0.00 3.20 2.25 0.65 9.54
sdg 121.60 8033.60 95.80 44.07 12.39 66.07 30.60 3654.40 33.80 52.48 12.47 119.42 0.00 0.00 0.00 0.00 0.00 0.00 2.60 8.77 1.91 97.24
sdh 122.00 6105.60 69.00 36.13 5.47 50.05 33.00 3748.00 34.80 51.33 16.94 113.58 0.00 0.00 0.00 0.00 0.00 0.00 2.20 5.00 1.24 42.20
sdi 117.00 6856.80 81.60 41.09 12.25 58.61 32.00 2612.80 19.80 38.22 10.80 81.65 0.00 0.00 0.00 0.00 0.00 0.00 2.60 8.46 1.80 91.18
sdj 1.60 8.00 0.00 0.00 13.12 5.00 31.20 2563.20 15.40 33.05 5.06 82.15 0.00 0.00 0.00 0.00 0.00 0.00 2.00 1.70 0.18 3.70
With settings
osd_deep_scrub_stride = 4194304
osd_scrub_load_threshold = 20
osd_scrub_chunk_min = 15
osd_scrub_chunk_max = 75
osd_max_scrubs = 3
I get a slight improvement only:
Device r/s rkB/s rrqm/s %rrqm r_await rareq-sz w/s wkB/s wrqm/s %wrqm w_await wareq-sz d/s dkB/s drqm/s %drqm d_await dareq-sz f/s f_await aqu-sz %util
dm-0 400.60 14686.40 0.00 0.00 19.93 36.66 25.20 1197.60 0.00 0.00 27.44 47.52 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 8.67 91.32
dm-1 362.60 10583.20 0.00 0.00 18.58 29.19 30.40 1742.40 0.00 0.00 23.32 57.32 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.45 98.14
dm-10 6.40 64.00 0.00 0.00 0.03 10.00 37.60 150.40 0.00 0.00 0.06 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.02
dm-11 76.60 1939.20 0.00 0.00 10.61 25.32 35.20 1885.60 0.00 0.00 22.32 53.57 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.60 29.06
dm-12 93.00 4178.40 0.00 0.00 16.04 44.93 4.40 96.00 0.00 0.00 0.68 21.82 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.50 19.04
dm-13 376.80 12716.00 0.00 0.00 15.90 33.75 27.20 1444.80 0.00 0.00 23.76 53.12 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6.64 97.76
dm-14 11.80 312.80 0.00 0.00 0.12 26.51 33.40 133.60 0.00 0.00 0.00 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.96
dm-15 7.00 430.40 0.00 0.00 0.17 61.49 46.20 184.80 0.00 0.00 0.06 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.94
dm-16 11.80 229.60 0.00 0.00 0.10 19.46 49.20 196.80 0.00 0.00 0.09 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 1.20
dm-17 4.00 39.20 0.00 0.00 0.10 9.80 29.60 118.40 0.00 0.00 0.00 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.72
dm-18 9.00 94.40 0.00 0.00 0.07 10.49 37.20 148.80 0.00 0.00 0.30 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.92
dm-19 7.40 70.40 0.00 0.00 0.11 9.51 56.80 227.20 0.00 0.00 0.02 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.04
dm-2 0.00 0.00 0.00 0.00 0.00 0.00 62.00 482.40 0.00 0.00 0.07 7.78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.16
dm-20 12.40 2375.20 0.00 0.00 0.23 191.55 314.20 1524.80 0.00 0.00 0.03 4.85 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 7.90
dm-21 321.00 9468.00 0.00 0.00 7.86 29.50 31.20 1630.40 0.00 0.00 11.69 52.26 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.89 42.84
dm-3 12.40 125.60 0.00 0.00 0.08 10.13 37.00 148.00 0.00 0.00 0.02 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.84
dm-4 374.00 13150.40 0.00 0.00 18.31 35.16 23.60 1100.80 0.00 0.00 12.42 46.64 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 7.14 96.70
dm-5 8.00 84.00 0.00 0.00 0.10 10.50 38.00 152.00 0.00 0.00 0.03 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00
dm-6 201.60 6619.20 0.00 0.00 11.56 32.83 3.40 108.80 0.00 0.00 2.29 32.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.34 58.24
dm-7 414.40 14476.00 0.00 0.00 21.99 34.93 10.80 235.20 0.00 0.00 5.19 21.78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9.17 98.00
dm-8 0.60 14.40 0.00 0.00 0.00 24.00 40.80 163.20 0.00 0.00 0.04 4.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.70
dm-9 478.00 17300.80 0.00 0.00 23.06 36.19 4.40 92.00 0.00 0.00 5.00 20.91 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 11.05 98.00
md127 5.00 64.80 0.00 0.00 0.04 12.96 133.80 2014.40 0.00 0.00 0.08 15.06 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 2.32
nvme0n1 33.60 822.40 0.00 0.00 0.09 24.48 252.40 3388.10 139.40 35.58 0.04 13.42 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 7.24
nvme1n1 62.60 3082.40 0.00 0.00 0.10 49.24 427.40 4272.90 177.20 29.31 0.03 10.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 13.70
sda 236.20 10613.60 126.20 34.82 15.19 44.93 11.80 1742.40 18.60 61.18 13.83 147.66 0.00 0.00 0.00 0.00 0.00 0.00 1.40 20.29 3.78 98.14
sdb 35.80 4166.40 56.80 61.34 10.03 116.38 4.20 96.00 0.20 4.55 0.81 22.86 0.00 0.00 0.00 0.00 0.00 0.00 0.80 0.25 0.36 19.04
sdc 118.20 6612.80 83.00 41.25 9.15 55.95 3.00 108.80 0.40 11.76 2.07 36.27 0.00 0.00 0.00 0.00 0.00 0.00 0.60 3.67 1.09 58.20
sdd 207.00 13152.00 166.60 44.59 13.92 63.54 15.80 1100.80 7.80 33.05 6.90 69.67 0.00 0.00 0.00 0.00 0.00 0.00 1.80 9.67 3.01 96.66
sde 55.80 1939.20 20.80 27.15 9.06 34.75 17.40 1885.60 17.80 50.57 7.51 108.37 0.00 0.00 0.00 0.00 0.00 0.00 1.80 1.44 0.64 29.04
sdf 221.40 12720.00 155.20 41.21 14.34 57.45 14.60 1444.80 12.60 46.32 10.51 98.96 0.00 0.00 0.00 0.00 0.00 0.00 1.40 11.71 3.35 97.70
sdg 234.60 14490.40 179.80 43.39 15.66 61.77 9.40 235.20 1.20 11.32 5.36 25.02 0.00 0.00 0.00 0.00 0.00 0.00 2.00 11.10 3.75 97.98
sdh 212.20 9470.40 109.20 33.98 4.05 44.63 19.40 1630.40 11.80 37.82 6.41 84.04 0.00 0.00 0.00 0.00 0.00 0.00 2.00 4.10 0.99 42.78
sdi 214.00 14684.00 186.60 46.58 14.46 68.62 10.00 1197.60 15.20 60.32 11.14 119.76 0.00 0.00 0.00 0.00 0.00 0.00 2.40 13.67 3.24 91.28
sdj 250.60 17296.80 227.00 47.53 18.76 69.02 3.60 92.00 0.80 18.18 5.72 25.56 0.00 0.00 0.00 0.00 0.00 0.00 0.80 16.25 4.73 98.00
`osd_max_scrubs` creates more reads per second (~120 -> ~220), but does not proportionately increase read throughput (as is expected on a spinning disk).
Overall, this looks like scrub operations are seek-bound.
Looking at `strace -fyp 640105 -e io_submit` output, for lines that involve my HDD `/dev/dm-12`:
io_submit(0x7fd5f6668000, 2, [{aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd55baa3000, iov_len=4096}], aio_offset=1654046691328}, {aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f378000, iov_len=24576}], aio_offset=1654046666752}]) = 2
io_submit(0x7fd5f6668000, 2, [{aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f262000, iov_len=4096}], aio_offset=1934563819520}, {aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f28b000, iov_len=4096}], aio_offset=1934563823616}]) = 2
io_submit(0x7fd5f6668000, 2, [{aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f3c7000, iov_len=28672}], aio_offset=2871307956224}, {aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f2d2000, iov_len=4096}], aio_offset=2871307984896}]) = 2
io_submit(0x7fd5f6668000, 2, [{aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f34a000, iov_len=8192}], aio_offset=4056494669824}, {aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f35f000, iov_len=4096}], aio_offset=4056494678016}]) = 2
io_submit(0x7fd5f6668000, 2, [{aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f3e3000, iov_len=53248}], aio_offset=1233895051264}, {aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f0c6000, iov_len=4096}], aio_offset=1233895104512}]) = 2
io_submit(0x7fd5f6668000, 2, [{aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f1e2000, iov_len=4096}], aio_offset=2155792314368}, {aio_data=0, aio_lio_opcode=IOCB_CMD_PREADV, aio_fildes=30</dev/dm-12>, aio_buf=[{iov_base=0x7fd54f303000, iov_len=24576}], aio_offset=2155792289792}]) = 2
This looks like it's reading the HDD all over the place, with small reads (`aio_offset`, `iov_len`).
Does Ceph scrubbing do a HDD seek for every object?
I had imagined that scrubbing might be able to read linearly through the disk (perhaps skipping larger gaps).
Is that wrong?
Thanks!
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