Re: Question about slow access to file information

[Eliot Moss <moss@cs.umass.edu>]

On 1/15/2023 3:38 AM, Christian Franke via Cygwin wrote:
> Eliot Moss via Cygwin wrote:
>> I have a separate drive mounted this way:
>>
>> d:/ /cygdrive/d ntfs binary,posix=0,user,noacl,auto 0 0
>>
>> One thing I use it for is to store backup files.  These tend to be 2 Gb
>> chunks, and there can be hundreds of them in the backup directory. (The drive
>> is 5Tb.)  The Windows Disk Management tool describes it as NTFS, Basic Data
>> Partition.
>>
>> Doing ls (for example) takes a very perceptible numbers of seconds (though
>> whatever takes a long time seems to be cached, at least for a while, since a
>> second ls soon after is fast).
> 
> The problem is the 'noacl' mount option and the fact that POSIX only offers the *stat*() functions 
> to retrieve file information. These functions always need to provide the full file information, even 
> if only a small subset is needed.
> 
> To determine the 'x'-permission bits in the 'stat.st_mode' field on a 'noacl'-mount, Cygwin reads 
> the first bytes of most files (all except *.exe, *.lnk, *.com). The 'x' bits are set if the file 
> starts with "#!" (script), ":\n" (?) or "MZ" (Windows executable).
> 
> On 'noacl' mounts, this behavior could be suppressed by 'exec' or 'noexec' mount options.

Interesting.  I removed the noacl from /etc/fstab and restarted all Cygwin processes.
The mount program now shows that drive without noacl.  It still takes surprisingly
long to ls if I have not done so recently.  The directory contains ~1200 files.

Further thoughts?

EM