ReFS on Windows 8.1/Server 2012 R2 and "ERROR 665" "The requested operation could not be completed due to a file system limitation"

ReFS on Windows Server 2012 R2/Windows 8.1 newly allows named streams (aka ADS) but only up to a limit of 128Kbytes.  If you copy, to an ReFS volume, a file with a named stream over this size limit you will get ERROR 665 (0x00000299): The requested operation could not be completed due to a file system limitation.

I discovered this copying 2.3M+ files from backups to a new ReFS volume on a system running Server 2012 R2.  All but 5 files copied without error.  The five which failed, with error 665, were all IE “favorites” (i.e., dinky files with extension “.url” formatted like an INI file).  Nothing funky-looking in their names (like odd Unicode characters, not that that should have mattered) or filepath length (not that that should matter either, for ReFS).  Took me awhile to figure it out—as of this writing there are no useful Google hits for this error number or message with the string “ReFS”—and also I believed that ReFS didn’t support named streams.  (But that limitation was lifted in Windows 8.1.)

Anyway, it turns out IE puts favicons in named streams and some of them are over 128Kb in length!  In my case, 5 out of thousands.

Since the Windows CopyFileEx and similar APIs copy named streams transparently the error message you receive from applications will have the file name, but not a stream name.

So this is one way to get a mysterious Error 665 “The requested operation could not be completed due to a file system limitation” when copying files to/restoring from backup to an ReFS file system.

(P.S.: Microsoft TechNet documentation on named streams on ReFS in Windows Server 2012 R2/Windows 8.1.)