What is the NTDiskQuotaSidCache.ndx file for?

A customer was looking for information about the file %LOCAL­APP­DATA%\Microsoft\Windows NT\Disk­Quota\NT­Disk­Quota­Sid­Cache.ndx. All they could find is this KB article.

The NT­Disk­Quota­Sid­Cache.ndx file is used by the Disk Quota control panel. It's a cache of SIDs and names, to improve performance of mapping SIDs to display names. You can delete it, and it will be regenerated the next time you run the Disk Quota control panel.

The customer never explained what they needed the information for, but they did thank us. (Or at least their liaison thanked us on the customer's behalf.)

Comments (12)
  1. Koro says:

    Why does that particular component need a cache? If the lookup is that slow, should not that cache be moved to a more global level?

    1. ta.speot.is says:

      Why does every component need that cache? If it doesn’t, should not that cache be moved to a more local level?

      Global solution to a local problem.

      1. xcomcmdr says:

        Why it would be slow only for the Disk Quota Control Panel, then ?

        1. ranta says:

          The documentation of IDiskQuotaControl::InvalidateSidNameCache looks like it deals with this cache. I presume the cache will apply to any program that uses IDiskQuotaControl.

          The quota UI displays the names of all accounts that own files in the volume, so I think the list tends to be longer than any ACL, and benefits more from a cache.

  2. Erik F says:

    That particular file has been around since at least Windows 2000, so my guess is that it’s not strictly required but isn’t hurting anything. I doubt that it needs a cache, but removing it would be a a use of resources that doesn’t really do much, considering the number of people who use the disk quota system in the first place!

  3. kantos says:

    You know when you can smell a customer doing something they probably shouldn’t… yeah this smells a LOT like that.

    1. CB says:

      Lol, looking for information = suspicion of wrong doing.

    2. Ray Koopa says:

      Probably one of those “Cleaner” tool writers looking for what temporary files they can remove.

  4. Joshua says:

    Probable explanation: We must account for every file being copied to roaming profiles for security audit reasons.

    1. Damien says:

      That would be a tad unusual considering the file is in %LOCALAPPDATA% so doesn’t form part of the roaming profile.

  5. “The customer never explained what they needed the information for”

    Curiosity. And having a customer support contract.

    Another case is that they have a malware infection (or think they had) and are now looking for suspicious files. Ouch!

  6. DWalker says:

    About caches — let’s apply one of Raymond’s favorite phrases: “What if everybody did this?”.

    Everything in the system could be cached, and then … performance goes down….

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