CODEWITHSUNDEEP
windowscreatefile
tcb
tcb

Reputation: 4604

CreateFile with FILE_FLAG_NO_BUFFERING but not FILE_FLAG_WRITE_THROUGH

Is it valid to call CreateFile with the FILE_FLAG_NO_BUFFERING flag but without the FILE_FLAG_WRITE_THROUGH flag? In what situations will this usage be considered useful?

FILE_FLAG_NO_BUFFERING indicates the OS shouldn't buffer the data, but the absence of FILE_FLAG_WRITE_THROUGH indicates the OS shouldn't write through to the disk. These seem like conflicting requirements, how can the OS not buffer and not write-through?

Upvotes: 1

Views: 3674

Answers (1)

mrok
mrok

Reputation: 185

According to a recent Microsoft blog post, the FILE_FLAG_WRITE_THROUGH flag "[tells] the disk controller to flush the data out of its internal cache."

The table at the bottom of that post further clarifies that there is "no hardware flush" when FILE_FLAG_NO_BUFFERING is set without FILE_FLAG_WRITE_THROUGH.

Also, as Eryk Sun previously noted in a comment, FILE_FLAG_NO_BUFFERING is not documented to immediately write file metadata to disk, whereas FILE_FLAG_WRITE_THROUGH is documented to immediately write file metadata to disk. This makes sense because the focus of FILE_FLAG_NO_BUFFERING is to avoid copying of file data into OS-owned buffers, which seems an unreasonable goal for file system metadata since that would necessitate restricting file systems to one specific, publicly defined on-disk layout of file metadata.

Upvotes: 1

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