A blog by Jose Barreto, a member of the File Server team at Microsoft.
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Question from a user (paraphrased): When we access a local file via loopback UNC path, is this the same as accessing via the local path? I mean, is "C:\myfolder\a.txt" equal to "\\myserver\myshare\a.txt" or I'll be using TCP/IP in any way?
Answer from SMB developer: When accessing files over loopback, the initial connect and the metadata operations (open, query info, query directory, etc.) are sent over the loopback connection. However, once a file is open we detect it and forward reads/writes directly to the file system such that TCP/IP is not used. Thus there is some difference for metadata operations, but data operations (where the majority of the data is transferred) behave just like local access.
This is my question!!! I was the one who asked!!!
By the way, since which Windows version is this true? I mean, is this a Windows Server 2012 new feature or things always has worked this way?
Is this for SMB 2.X AND SMB 3.0 both?
Also in theory, the ACL parsing when going local or loopback is different?
This optimization was introduced with Windows Server 2003 / Windows XP SP1 and is included in all versions after that.
This is for all versions of SMB.
Only reads and writes go through this optimized local path, so all ACL parsing follows the regular loopback path, along with all metadata operations.