Block Writes Across Net May Cause Performance Hit

• Microsoft Windows NT Server versions 3.5 and 3.51

SYMPTOMS

When you are performing RAW mode writes using the redirector, you may incur a performance hit from disk reads.

CAUSE

When the redirector sends a large block size across the net, it must divide the blocks into chunks less than 64K to fit into a server message block (SMB) packet. The redirector takes the size of the block and divides it by the maximum data size (less than 64K) and uses this size to send the blocks across the net. This result in an odd size being written to the disk on the server side.

When the server receives the block to write (for example, 30K) it must then write this information on page boundaries. On i386 and MIPS this is 4K, on the DEC Alpha this is 8K. In the process of writing this data, it will be forced to write information that is only part of a 4K or 8K page. In this case is must read the portion of the page that has not changed, thus causing the writes.

STATUS

Microsoft has confirmed this to be a problem in Windows NT version 3.51. This problem was corrected in the latest Windows NT 3.51 U.S. Service Pack. For information on obtaining the Service Pack, query on the following word in the Microsoft Knowledge Base (without the spaces):

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