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The other issue is that converting to thin provisioning isn't always easy. You might argue that if a host is using only 20% of its allocated storage, then the unused 80% should become available as free storage once a conversion is complete. That's easier said than done. For starters, the conversion process isn't that simple. Most vendors don't support an online or transparent conversion. Those that support this conversion depend on host-based tools to do so. More importantly, the use of block-level copy tools, such as volume manager mirroring, don't provide the necessary effect because they're designed to update every block on the target storage as a part of the mirroring or copy process. It doesn't matter if that block is unused on the source storage; if it's been allocated, it means it must be mirrored. The target storage then treats a written block as an "accessed" block and the effect of thin provisioning is lost. As a result, some vendors insist that to take advantage of thin provisioning, you need to use it for net-new storage or use conventional methods such as tar or cp to migrate the data.
Unfortunately, thin provisioning hasn't yet evolved to the level of content addressable storage where there's a timestamp associated with every block and it's "retired" depending on its last access time. Perhaps, someday, we'll have that functionality.
Thin provisioning isn't a revolutionary
| concept, but a novel (and promising) way to address allocation and performance. You must remember that it's not a magical solution to every problem in your storage world. When one of those turns up, I'll be sure to let you know.
This was first published in April 2008