CODEWITHSUNDEEP
linuxasynchronousiolinux-kernelaio
ARH
ARH

Reputation: 1395

Duplicated LBA request in Block Layer

I am using blktrace to understand Block Layer behavior in a multi-thread IO workload with high IO load. I can observe some duplicated LBA request in block layer. For example, I have only one write request to LBA 47568 in my original workload. However, blktrace shows 121 requests writing LBA 47568 !!! below is a portion of blkparse log which filter only I events (IO inserted onto request queue). I am trying to understand why this phenomena happen and how can I eliminate that. I picked noop IO scheduler and value 2 in /sys/block/sdX/nomerge to avoid IO merges. I do appreciate your help in advance.

  8 240  0     7539     0.073300123 32282  I  WS 47576 + 8 [a]
  8 240  5     4462     0.073303974 32281  I  WS 47568 + 8 [a]
  8 240 11     4462     0.073306337 32283  I  WS 47584 + 8 [a]
  8 240  6     4462     0.073307970 32284  I  WS 47592 + 8 [a]
  8 240 10     4462     0.073309010 32285  I  WS 47600 + 8 [a]
  8 240  5     4466     0.073311526 32281  I  WS 47568 + 8 [a]
  8 240  5     4469     0.073316215   175  I  WS 47568 + 8 [kworker/5:1]
  8 240  7     4462     0.073352932 32286  I  WS 47608 + 8 [a]
  8 240  7     4466     0.073354061 32286  I  WS 47568 + 8 [a]
  8 240  8     4468     0.073358935 32280  I  WS 47624 + 8 [a]
  8 240  4     4468     0.073360550 32279  I  WS 47616 + 8 [a]
  8 240  8     4472     0.073362145 32280  I  WS 47568 + 8 [a]
  8 240  5     4473     0.073362711 32281  I  WS 47632 + 8 [a]
  8 240  5     4477     0.073364177 32281  I  WS 47568 + 8 [a]
  8 240  0     7545     0.073407427 32282  I  WS 47640 + 8 [a]
  8 240  0     7549     0.073409589 32282  I  WS 47568 + 8 [a]

Upvotes: 3

Views: 340

Answers (1)

ARH
ARH

Reputation: 1395

Ok, I am gonna answer my question. It turns out that number of Outstanding IOs in the workload is larger than generic device driver queue size (/sys/block/sdX/device/queue_depth). Therefore, some requests failed to insert in this queue when it is full and cause such a phenomena.

Upvotes: 3

Related Questions