Which techniques are used by Facebook Haystack to optimize its photo access performance?

$($A$)$ It does not reduce each photo's metadata in the filesystem but keeps all such metadata in the main memory

$($B$)$ It does not store each photo as a separate file, but stores a few large files, each of which keeps many photos

$($C$)$ Haystack Cache may shelter write$-$enabled Stores from reads

$($D$)$ Index file is used for fast rebooting, since all needles have their corresponding index records