[memcached_extstore]extstore fail to take effects, has anybody meet the same questions?

Question

I'm trying the memcached_extstore and succeed in starting it, but when I try some 4KB writes ,I found that no data write to extstore,has anybody meet the same questions?

Here's my code Wapiti@node1:~/mydata/memcached-1.6.27$ sudo ./memcached -u root -m 6000 -I 2m -c 20000 -o ext_path=/users/Wapiti/mydata/ext_store:30G,ext_wbuf_size=32,ext_threads=10,ext_max_sleep=10000,slab_automove_freeratio=0.10,ext_recache_rate=0,ext_item_size=1024

I start Memcached on localhost:11211, in tulnet stats I can see descriptions about ext_store ,and I write a python program use simple "set","get" to monitor 4KB read/write, but the file ext_store is still blank, data was written to memcached but not the file ,I wonder why that happend

By the way, can anybody tell me how to test the performance of Memcached in the size of 4KB,or should I try other methods to test it? thanks a lot :D

I hope data can be written to the file /users/Wapiti/mydata/ext_store correctly