reviewed vcache for use by git-annex; found it wanting

doc/bugs/incremental_fsck_should_not_use_sticky_bit/comment_8_9f5acbc79b631a93d7cdf4ae37c07cab._comment

@@ -0,0 +1,19 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""vache"""
+ date="2015-02-15T16:35:12Z"
+ content="""
+<https://github.com/dmbarbour/haskell-vcache>
+
+It uses LMDB, which is a C library, and its PVar is a variable named by a
+bytestring, so it's essentially a key/value store where the values can be
+arbitrary Haskell data types. Since git-annex already has Keys, and most
+of the need for the database is to look up some cached value for a Key,
+this seems like a pretty good fit!
+
+Unfortunately, "A VCache file may be opened by only one process at a time,
+and only once within said process."
+
+But, git-annex needs multiple read-only access, as many git-annex processes
+can run concurrently.
+"""]]

doc/design/caching_database.mdwn

@@ -45,14 +45,6 @@ DB into memory. In the comments of
 other haskell database-like things, and found them all wanting, except for
 possibly TCache.
 
-TODO: This seems promising; investigate it:
-<https://awelonblue.wordpress.com/2014/12/19/vcache-an-acid-state-killer/>  
-It uses LMDB, which is a C library, and its PVar is a variable named by a
-bytestring, so it's essentially a key/value store where the values can be
-arbitrary Haskell data types. Since git-annex already has Keys, and most
-of the need for the database is to look up some cached value for a Key,
-this seems like a pretty good fit!
-
 ## case study: persistent with sqllite
 
 Here's a non-normalized database schema in persistent's syntax.