One of the topics I've often written and spoken about is thin provisioning. This series of 11 articles is an edited version of my thin provisioning presentation from Interop New York 2010. I hope you enjoy it!
If WRITE_SAME can be a semaphore for thin un-provisioning, what about TRIM? It sounds like a perfect fit, and has wider implementation to boot! Let’s take a deeper look.
First, I’d like to remind the reader that I’m not some specialist storage engineer. I am regular guy who wanted to know about this TRIM thing. I welcome corrections and feedback on this piece!
We should note that TRIM was invented for SSDs, not thin provisioning. TRIM lets an operating system tell a storage device that certain blocks of data are no longer used. Although this sounds like a thin provisioning tool, it’s not. TRIM was never intended to be used this way.
Even though Microsoft supports TRIM in the file system, and even though, hopefully, Apple will introduce TRIM support in OS X Lion, this doesn’t do what we need it to do for thin provisioning. In fact, TRIM is not anything for enterprise storage folks to be getting all that excited about.
One reason for is is that TRIM is an ATA command, and most enterprise systems are not connected to their storage using any kind of ATA-based protocol. Maybe there are SATA disks somewhere down the line, but, whether they use iSCSI, FC, SAS, or FCoE, the servers speak SCSI.
Even though there are SCSI analogs of of TRIM, these commands were really not designed for thin provisioning. They are designed for SSDs, which have a big performance problem: An SSD can delete data and write data just fine, but in order to update data, they actually have to read, delete, and write a big page of data. What happen is that, as an SSD gets full, it starts slowing down. And that’s a problem because people buy SSDs for speed. So TRIM is a way to tell the SSD, “Sometime in the future, you can go ahead and delete this if you feel like it,” and the SSD will take care of that later.
There’s great OS support for this, but unfortunately, this is not something that enterprise storage vendors are looking at. The only vendor (that I know of) that’s working on this, NetApp, did propose to T10 a mechanism to do thin provisioning based on TRIM. I don’t think it got anywhere. They say that it did, but I just don’t see it. HDS and EMC, seem to like UNMAP and SCSI, which are peripherally related, but I don’t know much about what they’re doing either. Perhaps they’ll let me know in comments on this post.