On Thu, May 20, 2010 at 2:23 PM, Miika Vesti <miika.ve...@trivore.com> wrote: > "I'm pretty sure that all SandForce-based SSDs don't use DRAM as their > cache, but take a hunk of flash to use as scratch space instead. Which > means that they'll be OK for ZIL use."
I've read conflicting reports that the controller contains a small DRAM cache. So while it doesn't rely on an external DRAM cache, it does have one: http://www.legitreviews.com/article/1299/2/ "As we noted, the Vertex 2 doesn't have any cache chips on it as that is because the SandForce controller itself is said to carry a small cache inside that is a number of megabytes in size." > "Another benefit of SandForce's architecture is that the SSD keeps > information on the NAND grid and removes the need for a separate cache > buffer DRAM module. The result is a faster transaction, albeit at the > expense of total storage capacity." Again, conflicting reports indicate otherwise. http://www.legitreviews.com/article/1299/2/ "That adds up to 128GB of storage space, but only 93.1GB of it will be usable space! The 'hidden' capacity is used for wear leveling, which is crucial to keeping SSDs running as long as possible." My understanding is that the controller contains enough cache to buffer enough data to write a complete erase block size, eliminating the need to read / erase / write that a partial block write entails. It's reported to do a copy-on-write, so it doesn't need to do a read of existing blocks when making changes, which gives it such high iops - Even random writes are turned into sequential writes (much like how ZFS works) of entire erase blocks. The excessive spare area is used to ensure that there are always full pages free to write to. (Some vendors are releasing consumer drives with 60/120/240 GB, using 7% reserved space rather than the 27% that the original drives ship with.) With an unexpected power loss, you could still lose any data that's cached in the controller, or any uncommitted changes that have been partially written to the NAND I hate having to rely on sites like Legit Reviews and Anandtech for technical data, but there don't seem to be non-fanboy sites doing comprehensive reviews of the drives ... -B -- Brandon High : bh...@freaks.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
