[Sheepdog] [PATCH v5 06/17] farm: add sha1_file operations

[Liu Yuan]

On 01/05/2012 05:04 AM, MORITA Kazutaka wrote:

> We can consider two options here:
> 
>  a) calculate SHA1 after compressing the input data
>  b) calculate SHA1 directly
> 
> I guess you assume that a) is faster than b), but it's not obvious to
> me.


I was wrong, I wrote a test  and it shows that it's a) slower than b),
but we would gain storage reduction from the compressed content.

I simply copied that sha1_file_write() does in the test file. The result
is bellow, for a object 4M with "010101010101..." content which is ideal
for compression.

tailai.ly at taobao:~$ time ./a.out 1
sheepdog/store/0/obj/00000001/007c2b2500000001

real	0m0.062s
user	0m0.060s
sys	0m0.000s
tailai.ly at taobao:~$ ll -h store
-rw-r--r-- 1 tailai.ly tailai.ly 4.0K 2012-01-05 11:36 store
tailai.ly at taobao:~$ time ./a.out 0
sheepdog/store/0/obj/00000001/007c2b2500000001

real	0m0.036s
user	0m0.030s
sys	0m0.000s
tailai.ly at taobao:~$ ll -h store
-rw-r--r-- 1 tailai.ly tailai.ly 4.0M 2012-01-05 11:37 store

it shows that we get a good size reduction, trading off the CPU cycles.
I also tried vmlinuxz (4.2M) on my laptop, and compressed one is 10x
slower than the uncompressed.for such a object with random content, we
both burn cycles and lose size reduction.

With this numbers, I am not sure which one, either cpu cycles and
storage space, is more important than other. I am okay to drop
compressed logic.

Thanks,
Yuan