[Disksim-users] disk cache management

[hust_mb]

Hi,all

In my experiment I found a very strange thing: No write hit in on-board disk cache!
I use the validate trace as following:

W Hit    5568004   8   1464.00000  53368.00000            ---request 1--
W Hit    5568012   8   1323.00000  54353.00000            ---request 2--
W Hit    1505868   4   1098.00000  32082.00000            ---request 3--
W Hit    1505868   4    879.00000  39461.00000            ---request 4--

# ./disksim cheetah9LP.parv file_1 validate trace 0

But no hit found in the file_1. Theoretically there should be a write combine with request 1 and 2. Also a write hit with 4 after 3. But indeed there is no.

I tracked the  disk_buffer_select_write_segment and  disk_release_hda function. There is no conserve of write although I use the fast write mode. A new request own a segment for write and release it after completion.

And I increased the segment counts in the cheetah9LP.diskspecs. There is litter increase(less than 1%) about the cache hit ratio! (with command ./disksim cheetah9LP.parv file_1 validate cheetah9LP.trace 0)

Does this really the cache management for modern disk? 

I think the fast write mode enabled can conserve more small write and then write them optimizely (sometime sequentially write or with freeblock scheduling policy).

Thank you for your discussion.
------------------  
mb
2007-09-02
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