well, turning on NTFS compression is not an good idea and this for several reasons:
1. database engines (at least Oracle) did allocate disk space at creation or extension time of a datafile. This space is allocated and filled with a pattern which normally allows very good compression rates. During this free space gets filled the compression rate will often drop with the result that the originally allocated space might not be available later. Nobody expects that allocated space is not available.
These errors may result in a corrupted datafile.
2. the file compression will happen after closing the created file. This means that at least for a single datafile the disk freespace must be sufficient to keep the entire uncompressed file.
3. as a result of 2. and the fact that the compression will run in place (a file block is compressed and his compressed data is written into the same block. As soon as there is a completely free block it will be added to NTFSs freelist) compression will lead to highly fragmented volumes.
4. file compression will automatically allocate Paged- and Nonpaged Pool Bytes by the compression driver in the operating system. The amount of bytes allocated is depending on the number and the size of the openen compressed files. If running in a 32 Bit operating system
Pool Bytes may become a serious bottleneck in your system. If running out of Pool Bytes several functions in the operating system and every application on your server may missbehave. I have seen installations where it was no longer possible to connect addtional database clients and connected clients get errors after one of the pools was exhausted.
5. file compression is not supported officially by one of the databases I know.
regards
Peter
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