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Handling placeholders

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Guidance for minifilters that implement placeholders

All virtualization implementations that use placeholders must set the FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS file attribute on these placeholders. This attribute indicates that:

  • Reading such files from below the owning minifilter might return 0s, and
  • Reading such files from above the owning minifilter entails the extra cost of furnishing the data.

A minifilter can remove this attribute from the placeholder once the entire data is available locally.

All minifilters that implement placeholders must be in the HSM Load Order Group.

Guidance for all minifilters

Filters must not issue targeted reads/writes from filter instances below the HSM Load Order Group for files that have the attribute FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS set. This situation could lead to file data corruption. A minifilter developer could try to circumvent this situation by issuing an IO request to the top of the stack, but such a request could lead to deadlocks.

Filters should also not issue reads and writes on placeholder files that have the FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS attribute set from above the HSM Load Order Group. Such a read or write causes unnecessary hydration when no user application has requested the file data.

A minifilter shouldn't issue reads/writes on intercepting attribute-only opens. Such read/writes can cause deadlocks as certain implementations don’t expect attribute-only opens to be converted to data access operations. Further, such read/writes defeat the purpose of a minifilter checking for FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS so it can avoid the two aforementioned situations.

Thus, it's recommended that all minifilters shouldn't issue reads/writes on files that have FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS set.