On 01/25/2013 03:11 PM, Edward Ned Harvey (openindiana) wrote: >> From: Christopher Chan [mailto:christopher.c...@bradbury.edu.hk] >> >> :-D I'm here to entertain since I have not been able to spring for a ssd >> for use as a slog. :-D > > LOL, you mean you have a HDD slog device? :-D > It's actually very surprising how well that works, especially if you have a > high rpm drive. Because most of the wasted access time in a HDD is waiting > for head seek time. Usually, the rotational latency is like 1ms or so, so > it's irrelevant compared to the head seek, but if you do a good job of > eliminating the head seek time, then the rotational latency becomes > completely relevant, or even dominant. If your drive is used only for slog, > guess what ... ZFS does a pretty good job of keeping the ZIL clustered > together in tightly grouped tracks, so you've done a pretty good job of > eliminating the seek. ;-) > > If you let the ZIL sit on main pool, it will both be adversely affected by > other reads and writes ... And it will also adversely affect other reads and > writes, mutually. And it's more multiplicative rather than additive. > Because you now have large random seek times, and cache flushes, so all your > IO optimization techniques get messed up.
Agreed, short-stroking a 15k HDD to something like 1/100th of its capacity actually gives you really good transactional throughput. Considering a 15k 300GB SAS drive is comparatively cheap nowadays, this gives you 3GB of slog (more than enough) at fairly low latency (1000 iops is achievable). -- Saso _______________________________________________ OpenIndiana-discuss mailing list OpenIndiana-discuss@openindiana.org http://openindiana.org/mailman/listinfo/openindiana-discuss
