xfs seems performance lower when long time seq write into ssd

[çå <wang.yong@xxxxxxxxxxx>]

Hi All,
Recently, I meet a  strange quetion. I have done seq write
(blocksize=1m) into mount folders.
If total bytes is small, the avg write rate is 370MBs (little files :
 4MB * 256 * 60 )
If total bytes is big, the avg write rate is 180MBs  (a lot of files :
  4MB * 256 * 600)
the raw ssd benchmark, seq write is 400MBs.

Can anybody help to explain it?  argument is wrong, or something else.

Below argument list:

Hardware:        Intel 3750 ssd  480GB  (sata3)
centos:             7.3  x86_64
kernel:              3.10.0-514.el7.x86_64
software:          mkfs.xfs
mount feature:   /dev/sdb1  type xfs
(rw,noatime,attr2,inode64,logbsize=256k,noquota)

xfs_info /dev/sdl1 :
meta-data=/dev/sdl1              isize=512    agcount=4, agsize=28975477 blks
         =                       sectsz=4096  attr=2, projid32bit=1
         =                       crc=1        finobt=0 spinodes=0
data     =                       bsize=4096   blocks=115901905, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=1
log      =internal               bsize=4096   blocks=56592, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

Tks and Rgds