data from blktrace: read speed V.S. write speed

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Special Forums Hardware Filesystems, Disks and Memory data from blktrace: read speed V.S. write speed
# 1  
Old 10-26-2009
data from blktrace: read speed V.S. write speed

I analysed disk performance with blktrace and get some data:
  8,3    4     2141     2.882115217  3342  Q   R 195732187 + 32 [cat]
  8,3    4     2142     2.882116411  3342  G   R 195732187 + 32 [cat]
  8,3    4     2144     2.882117647  3342  I   R 195732187 + 32 [cat]
  8,3    4     2145     2.882118965  3342  D   R 195732187 + 32 [cat]
  8,3    4     2149     2.884145702     0  C   R 195732187 + 32 [0]

  8,3    4    43622     5.305507398  3436  Q   W 17805895 + 8 [bash]
  8,3    4    43623     5.305508277  3436  G   W 17805895 + 8 [bash]
  8,3    4    43624     5.305508574  3436  I   W 17805895 + 8 [bash]
  8,3    2      414     5.306695591     0  D   W 17805895 + 16 [swapper]
  8,3    2      418     5.307020642     0  C   W 17805895 + 16 [0]

And, d2c time of one read request is 0.002026737s. On the other hand, d2c time of one write request is 0.000325051s. It show: read time > write time. Is it right? why? I have no idea.
# 2  
Old 10-26-2009
The write operations are almost certainly getting cached somewhere, most likely in the kernel page cache or the cache on the disk(s) themselves depending on your OS and hardware setup.
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