Large-sector drives

Supporting large-sector (>4K) drives was the topic of Ric Wheeler's fairly brief session at the 2014 LSFMM Summit. Supporting drives with 4K sectors only came about in the last few years, but 4K sectors are not the end of the story.

Wheeler asked the drive vendor representatives if they chose 4K as their sector size willingly, or simply because it is the largest size supported by operating systems. While no one directly answered the question, it was evident that drive vendors would like more flexibility in sector sizes. While they may not want sectors as large as 256M (jokingly suggested by Martin Petersen), storage arrays have been using 64K and 128K sectors internally for some time, Wheeler said.

Dave Chinner asked which layers would need to change to handle larger sector sizes, and suggested that filesystems and partition-handling code were likely suspects. He also said that the ability to do page-sized I/O is a fundamental assumption throughout the page cache code. Jan Kara mentioned reclaim as another area that makes that assumption. Linux pages are typically 4K in size.

One way to deal with that would be with chunks, as IRIX did with its chunk cache, Chinner said. It was a multi-page buffer cache that was created to handle exactly this case. But, he is not at all sure we want to go down that path. Petersen mentioned that there is another commercial Unix that has a similar scheme.

There could also be a layer that allows for sub-sector reads and writes. Though, it would have to do read-modify-write cycles to write smaller pieces, which would be slow, Chinner said.

Unlike the advent of 4K drives, the industry is not pushing for support of larger sector sizes immediately, Wheeler said. There is time to solve the problems correctly. The right approach is to handle sectors that are larger than the page size first, then to build on that, Ted Ts'o said.

[ Thanks to the Linux Foundation for travel support to attend LSFMM. ]