Re: [9fans] Flash

[erik quanstrom <quanstro@quanstro.net>]

> 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