Commits on Sep 21, 2021

1. delay storage operations for an extent that is transitioning from uni… …

   …nited to initialized state
   
   When a file extent that is marked as uninited is written to, all sectors
   corresponding to the extent are written, including both the data from the
   write request and any zeroed area(s) surrounding it. This presents a problem
   when subsequent storage requests are issued on behalf of the extent. Such
   requests may be made without first waiting for completion of the initializing
   write. This property of the pagecache - that requests are issued without first
   waiting for completions for prior requests - is typically not an issue, for
   read data is cached and not re-read after a write, and writes are posted from
   the same pagecache buffer, making the order of write completions
   irrelevant. But the areas of an extent outside of the initial write request,
   which are written with zeros, could be subsequently read into the cache (or
   pages overwritten as a whole) prior to the zero writes completing. This has
   been found to lead to spurious inconsistent data being read - namely, areas
   that should be zero are read and found to contain nonzero data.
   
   To ameliorate this condition, the uninited flag has been changed into an
   optional record that becomes allocated when a write is first posted to an
   uninited extent. When subsequent storage requests are made to the extent
   before the initializing writes complete, they are added to a queue within the
   uninited record. On completion of the initializing writes, the queued
   requests are then dispatched, and the uninited state is set to 'initialized',
   indicating that further operations on the extent should occur normally, as if
   uninited was not set. (Given that extent references are not tracked and that
   the extents themselves may be deallocated at any time, the uninited completion
   never refers to the extent itself, and the record exists for the lifetime of
   the extent - thus the uninited 'initialized' state having the same semantics
   as a null uninited field.)

   

   wjhun committed Sep 21, 2021
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