· Feb 2, 2016 1m read

Cache' databases as UNIX sparse files

Some third party backup products may by default restore CACHE.DAT files as UNIX sparse files when there are trailing zeroes in the backup file.

The support for sparse files vary from UNIX distribution and file system types.  Sparse files attempt to use file system space more efficiently when blocks allocated to the file are mostly empty similar to thin-provisioned storage.  The file system transparently converts metadata representing empty blocks into "real" blocks filled with zero bytes at runtime. The application is suppose to be unaware of this conversion.

The Problem

It has been reported to InterSystems in several occassions upon using some restored database CACHE.DAT files as sparse files may be identified by Cache' as corrupt databases.  The remedy in this case was to change the backup software default to not create sparse files.

The Recommendation

Due to the issues encountered and potential for inconsistency among file system types and platforms, it is recommended by InterSystems to not use sparse files at this time.  This post will be updated when there is additional information available.

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