#3725 HIGH First D: JFFS2 needs more aggressive garbage collection

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#3725: JFFS2 needs more aggressive garbage collection
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 Reporter:  wmb at firmworks.com  |       Owner:  dwmw2                            
     Type:  defect             |      Status:  new                              
 Priority:  high               |   Milestone:  First Deployment, V1.0           
Component:  kernel             |     Version:  Development build as of this date
 Keywords:  JFFS2              |    Verified:  0                                
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 Pursuant to #3247, Quanta provided us with a NAND FLASH image which
 appears not to have any obvious errors, but which takes a long time to
 mount.  The initial mount under OFW takes about 40 seconds and the
 subsequent mount under Linux takes about 50 seconds.  (Compare with
 typical 5-second mount times for fresh JFFS2 images from OS builds.) dwmw2
 and wmb both have copies of this image.

 The image resulted from Quanta's long-run testing, which includes
 writing/deleting/rewriting several files of various sizes many times.

 This image has 800K dirent nodes and 6M data nodes, compared to 25M
 dirents and 300K data nodes for a fresh image.  All but one of the blocks
 have summaries, so lack of summaries cannot account for the slowness.  The
 image occupies 8003 of the approximately 8192 (minus bad blocks) on the
 NAND, but I suspect that the amount of current (non-obsolete) data is far
 less.  If this image is like another long-run image that I analyzed some
 time ago, many of the nodes are  obsolete and there are lots of
 consecutive small nodes that could be coalesced.

 dwmw2 has offered some ideas for optimizing JFFS2's garbage collection
 process; it seems like a good time to implement some of those idea.

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Ticket URL: <https://dev.laptop.org/ticket/3725>
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