How does srv.sys decide on raw vs core mode depending on the underlying filesystem?

Question

Hello,

We are developing a file system for Windows using IFS Kit. We started to investigate a performance problem which caused our file system I/O to be much slower when shared over the network. After looking at it with FileMon and TCPView from Sysinternals we found out that if a NTFS/FAT was shared, the SMB client and server were transferring I/O in 60K blocks, while while sharing our file system it used 4K blocks.

These two block sizes correspond to the SMB "core" and "raw" modes - this is explained here by Microsoft.

The problem is that we cannot figure out what in our file system causes the windows share server (srv.sys) to choose core mode (4K) for our file system and raw mode (60K) for NTFS and FAT.

Even hints at what to check are welcome.

KIV

Answer 1

From the support page:

When you use Windows NT Explorer to copy a file from the client to a remote computer, data is typically transfered in Core mode in 4 KB blocks.

Have you tried this from a command line?

Answer 2

The issue was resolved by setting the FO_CACHE_SUPPORTED flag in the file system object.