Disk cache will not be trimmed after using 90%

[YunhuiChen]

Describe the bug (描述bug)

Disk cache will not be trimmed after using 90%
To Reproduce (复现方法)
1、test vdbench
fsd=fsd7,anchor=/home/nbs/failover/test7,depth=1,width=10,files=10,sizes=(10m,0),shared=yes,openflags=o_direct
fwd=fwd1,fsd=fsd*,threads=10,xfersize=(512,20,4k,20,64k,20,512k,20,1024k,20),fileio=random,fileselect=random,rdpct=50
rd=rd1,fwd=fwd*,fwdrate=max,format=restart,elapsed=120000,interval=1
2、

3、

Expected behavior (期望行为)

Versions (各种版本)
OS:
Compiler:
curve-mds:
curve-chunkserver:
curve-snapshotcloneserver:
curve-sdk:
nebd:
curve-nbd:

Additional context/screenshots (更多上下文/截图)

[wuhongsong]

The possible reasons for this phenomenon are the speed is slow upload to S3。 so after the vdbench is stop ，This phenomenon will disappear

[wuhongsong]

need repetition