Linux disk performance.

["Manish Regmi" <regmi.manish@gmail.com>]

Hi all,
      I was working in one application that requires heavy disk
writes, I noticed some inconsistencies in the write timing.
We are using raw reads to bypass filesystem overhead.

Firstly i tried open("/dev/hda",O_RDWR) i.e without O_DIRECT option.
I saw that after some writes 1 write took too much time.

the results are for writing 128KB data in MIPS 400mhz
sequence channel time (in microseconds)
0              1          1675
0              2           1625
0              3          1836
...
0               16       3398
0               63	1678
1               0         1702
1               1          1845
.....
3                46      17875              // large value
...
4               13      17142              ///
...
4              44      18711            /// large value

Is this behaviour ok?
I beleive this is due to deep request queue.

But when i used O_DIRECT. I got a little higher write times but it
also had such time bumps but at smaller rate.
-----------------------------------------
0              0       	3184
0	       1  	3165
0	       2	3126
...
0             52       10613                // large value
0             60        19004               //  large value

results similar with O_DIRECT|O_SYNC


Can we achieve smooth write times in Linux?

I am using 2.6.10 the results are moreover same (i dont mean
numerically same but i am getting thiming difference) in both P4 3 GHZ
512MB ram and MIPS. Disk is working in UDMA 5.

-- 
---------------------------------------------------------------
regards
Manish Regmi

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UNIX without a C Compiler is like eating Spaghetti with your mouth
sewn shut. It just doesn't make sense.