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Aligning filesystems to an SSD's erase block size

Feb 23, 2009, 14:03 (0 Talkback[s])
(Other stories by Theodore Ts'o)

"It turns out this is much more difficult than you might first think -- most of Linux's storage stack is not set up well to worry about alignment of partitions and logical volumes. This is surprising, because it's useful for many things other than just SSD's. This kind of alignment is important if you are using any kind of hardware or software RAID, for example, especially RAID 5, because if writes are done on stripe boundaries, it can avoid a read-modify-write overhead. In addition, the hard drive industry is planning on moving to 4096 byte sectors instead of the way-too-small 512 byte sectors at some point in the future. Linux's default partition geometry of 255 heads and 63 sectors/track means that there are 16065 (512 byte) sectors per cylinder. The initial round of 4k sector disks will emulate 512 byte disks, but if the partitions are not 4k aligned, then the disk will end up doing a read/modify/write on two internal 4k sectors for each singleton 4k file system write, and that would be unfortunate."

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