#9688 NORM 1.5-F11: terrible SD write performance

[Zarro Boogs per Child]

#9688: terrible SD write performance
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 Reporter:  dsd       |                 Owner:  wad          
     Type:  defect    |                Status:  new          
 Priority:  normal    |             Milestone:  1.5-F11      
Component:  hardware  |               Version:  not specified
 Keywords:            |           Next_action:  diagnose     
 Verified:  0         |   Deployment_affected:               
Blockedby:            |              Blocking:               
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 Our system feels slow and our syncs take ages because of terrible SD card
 write performance. I am testing on a system which is marked as having a
 sandisk C24G

 To reproduce: boot a system, press ctrl+alt+f2, run "sync" at bash prompt.
 It will take 5-10 seconds.

 blktrace shows that only ~100kb of data is being written in those 5-10
 seconds - approximately 12kb/sec.

 I traced this down to the sdhci level. The individual small, scattered
 writes that are being performed are often taking 400-600ms to complete
 (even just for a 2 sector write).
 I went as far to check that that delay definitely occurs at the point
 where the sdhci command has been dispatched and before the interrupt
 arrives. (there is no measurable delay in the lead-up to command
 submission, or the processing of the interrupt)

 I will attach some logs showing the write access pattern and the delays,
 and I will write a C program to reproduce this slow set of writes and
 hopefully act as a test case.

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Ticket URL: <http://dev.laptop.org/ticket/9688>
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