Do Solid State Disks Need Defragging?

By Alex Handy

July 15, 2008 —  The standard defragmentation procedures of most IT departments are not unlike the janitorial policies. A steady flow of cleaning supplies and work is required every day, while occasional mishaps require special attention. But just like the striking janitors of Silicon Valley, the defragmentation market is staring down a crisis of its own: Does the emergence of solid state disks (SSD) in servers eliminate the need for defragging?

At first glance, most experts will state that SSDs do not need to be defragged. Mike Karp, Systems Management News storage columnist, pointed out that there are two kinds of SSD and that defragmenting either is not needed.

“There is no conceivable reason to defragment either. SSDs based on RAM will not be adversely affected by defragging unless your concept of defragging involves the use of a hammer,” said Karp. “Flash-based SSDs are all also unlikely to be harmed, but there is a caveat: Because the lifetime of flash memory is typically limited by the total number of writes the memory can accept, and because defragging actually does a rewrite in the process of making data contiguous, a defragmenting procedure will shorten the life of the flash memory by lessening the number of writes that the chip can accept.”

While Karp is not alone in his view of defragging’s usefulness in SSDs, there are those who think otherwise. Most notable among the supporters of defragging SSD is Diskeeper, a Burbank, Calif.-based company that offers numerous defragmentation tools for standard hard drives and storage arrays.
 
Gary Quan, senior software architect at Diskeeper, acknowledged that flash-based SSDs have a lifespan, and therefore shouldn’t be defragmented as often as typical hard drives. But that’s not to say he hasn’t seen benefits from defragmentation in SSDs.

“What we found out is that fragmentation can really hurt the I/O bandwidth on SSDs. There isn’t that much of a performance degradation until the files become heavily fragmented. But there may be an increase in the number of I/O operations that have to occur. That’s not where we’re seeing the degradation. We’re seeing that during write activity. When the free space is badly fragmented, the writes can see 30 to 40 percent degradation in performance,” said Quan.

“[Let’s say that] files that are on a hard disk drive are in 10 fragments. On an SSD, you leave it alone because if you start moving that file, it’ll decrease the lifetime of the drive. On the free-space side, we have to be smarter about how we defrag that, but we want to take into consideration the lifespan of these SSDs. Files that aren’t getting accessed very often ... we want to put them in a location that’s more advantageous for the SSD. There are several things we’re implementing. There’s something on the write I/O bandwidth that’s occurring there. We’ve been talking with some I/O manufacturers and they seem to understand this,” said Quan. Diskeeper is currently working on new software specifically designed for defragmenting SSDs.

Why Defrag?
Quan’s assurances that SSDs need to be defragmented, even with the caveat that it’s the free space that necessitates the practice, is not a popular one. Hal Woods, distinguished technologist in Hewlett-Packard’s Storage Platforms Group Chief Technologist Office, isn’t sure that defragging SSD helps.

“The question is help what? SSDs deploy techniques called 'wear leveling' that will optimize the way data is stored into the flash memory to help improve performance and spread the data more evenly across the underlying flash memory to prevent (or at least delay) the wear-out,” said Woods. “Because SSDs do not have the motors, heads and bearings of an HDD or the related access delays associated with moving the heads and waiting for the data to come under the head to access data—the problem that the defragging is supposed to solve—is not present. In all likelihood, defragging an SSD will not help the SSD to perform better or last longer and it could be the case that the defrag process, if run often enough, will do enough writes to the SSD to somewhat shorten the life of the SSD. Because there are a number of vendors, each with their own wear-leveling approaches, a given SSD may get slightly worse or slightly better performance or endurance from defragging.”

Woods did see a scenario in which defragmentation might help, but it’s a different vision than that of Diskeeper. “Another aspect to consider is the benefit of defragging at the file system level. Defragging, independent of the underlying device, will make for more efficient space allocation algorithms in an environment where files are frequently created and deleted, and this could translate into some efficiencies in the I/O subsystem where large blocks of data are written instead of smaller blocks. This may help both HDD and SSDs perform better and last longer.”

Michael Cornwell, lead technologist of Flash and SSD at Sun Microsystems, thinks that Quan’s insistence that free space needs to be defragmented is on target, but that a third-party tool won’t be needed for such tasks.

“The defragmenting of free space on the SSD isn’t the issue. Most current and future generation SSDs will do defragmentation in the background as part of the wear-leveling system. The issue with free space on SSDs has to do with the device knowing what logical blocks are in use by the OS and file system. If the OS can notify the SSD of which blocks [that] are no longer in use, the device can become more efficient with wear-leveling and defragmentation. There is work in several storage protocol workgroups to develop a standard method for a free logical block notification,” said Cornwell.

Clod Barrera, distinguished engineer IBM systems storage, said that Cornwell is on the correct path of reasoning. He said that any data management practices that would increase the speed of access to SSDs would likely become part of the equipment, not part of a third-party defragmentation tool.

“When you go to SSDs, all of these issues kind of go away. What you have in a SSD is an addressable memory space and what you’re doing is you’re mapping a virtual disk onto this memory space. There’s no penalty for having a fragmented disk. In fact, you really have no idea what the real spreading of the data is in a solid-state environment. I can’t imagine what usage environment Diskeeper might be referring to that would require defragmentation,” said Barrera.



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