Hi, I created 40 osds ceph cluster with 8 PM863 960G SSD as journal. One
ssd is used by 5 osd drives as journal.   The ssd 512 random write
performance is about 450MB/s, but the whole cluster sequential write
throughput is only 800MB/s. Any suggestion on improving sequential write
performance? thanks.

Testing result is here:
rados bench -p libvirt-pool 10 write --no-cleanup
Maintaining 16 concurrent writes of 4194304 bytes to objects of size
4194304 for up to 10 seconds or 0 objects
Object prefix: benchmark_data_redpower-sh-04_16462
  sec Cur ops   started  finished  avg MB/s  cur MB/s last lat(s)  avg
lat(s)
    0       0         0         0         0         0           -
0
    1      15       189       174   695.968       696   0.0359122
0.082477
    2      16       395       379   757.938       820   0.0634079
0.0826266
    3      16       582       566   754.601       748   0.0401129
0.0830207
    4      16       796       780   779.934       856   0.0374938
0.0816794
    5      16       977       961   768.735       724   0.0489886
0.0827479
    6      16      1172      1156   770.601       780   0.0428639
0.0812062
    7      16      1387      1371   783.362       860   0.0461826
0.0811803
    8      16      1545      1529   764.433       632    0.238497
0.0831018
    9      16      1765      1749   777.265       880   0.0557358
0.0814399
   10      16      1971      1955   781.931       824   0.0321333
0.0814144
Total time run:         10.044813
Total writes made:      1972
Write size:             4194304
Object size:            4194304
Bandwidth (MB/sec):     785.281
Stddev Bandwidth:       80.8235
Max bandwidth (MB/sec): 880
Min bandwidth (MB/sec): 632
Average IOPS:           196
Stddev IOPS:            20
Max IOPS:               220
Min IOPS:               158
Average Latency(s):     0.081415
Stddev Latency(s):      0.0554568
Max latency(s):         0.345111
Min latency(s):         0.0230153

my ceph osd configuration:
sd_mkfs_type = xfs
osd_mount_options_xfs = rw,noatime,inode64,logbsize=256k
osd_mkfs_options_xfs = -f -i size=2048
filestore_max_inline_xattr_size = 254
filestore_max_inline_xattrs = 6
osd_op_threads = 20
filestore_queue_max_ops = 25000
journal_max_write_entries=10000
journal_queue_max_ops=50000
objecter_inflight_ops=10240
filestore_queue_max_bytes=1048576000
filestore_queue_committing_max_bytes =1048576000
journal_max_write_bytes=1073714824
journal_queue_max_bytes=10485760000
ms_dispatch_throttle_bytes=1048576000
objecter_infilght_op_bytes=1048576000
filestore_max_sync_interval=20
filestore_flusher=false
filestore_flush_min=0
filestore_sync_flush=true
journal_block_align = true
journal_dio = true
journal_aio = true
journal_force_aio = true
osd_op_num_shards=8
osd_op_num_threads_per_shard=2
filestore_wbthrottle_enable=false
filestore_fd_cache_size=1024
filestore_omap_header_cache_size=1024
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