> i don't understand this. your "worm" is magnetic disk, right? > why would you put the ssd in the slow half (the worm) > instead of the fast half (the cache)?
the current setup is that the worm and the "cache" are the same speed (in fact, the cache is slower because it's on two disks rather than 4). i found it was needlessly expensive to copy data from the worm to cache, so the only data that makes it to the cache are dirty blocks. it's used as a write buffer, not a cache. http://www.quanstro.net/plan9/disklessfs.pdf. if i simply reenabled the cache and replaced the two hard drives with ssd, i don't think we'd see much performance increase as we're not thrashing the ram cache yet. and for heavy write loads with a caching appliance, i think that 8 disk-limited disks would compete well with 2 sata-limited drives. i think one would get better bang/buck by replacing the worm drives with a greater number of smaller hard drives; ssds would be even better. on the other hand, a fusion i/o device would be a compelling reason to reinstitute a true cache. > similarly, someone on this thread said they'd use ssd > for just the arenas (which are mostly linear access), > when if i had to make the choice i would use it for just > the index (which is mostly random access and would > benefit more from dropping the seek penalities). i guess the $640 question is, how good is that wear leveling. - erik
