On Mon, Aug 10, 2009 at 12:44 PM, Bob Friesenhahn < [email protected]> wrote:
> Recent SSDs typically accumulate (coalesce) multiple writes in the >> >> on-board DRAM to flush these to NAND at a later time, so multiple >> writes are hardly an issue. >> > > If the SSD buffers the writes in DRAM then the postponed writes written via > NFS COMMIT become a total non-issue. I am hardly an authority on the subject, but in my characterization of ZFS/NFS performance with a battery backed, RAM-based slog, I found that the delay from pushing data across the PCI-X bus alone during COMMIT flush had a significant impact on NFS single stream write performance. I imagine this delay exists to a certain degree on all interconnect types (SATA, PCI-E, etc) even if the slog itself were infinitely fast. In the extreme case (say, multi-megabyte COMMITs) one can waste a good chunk of both the bus (or slog) bandwidth and the network bandwidth--the bus/slog is idle when the network is active, and the network is idle when the bus/slog is active. Whether or not it makes sense to take such aggressive measures to mask latency in specific cases is, of course, a separate question. Kaya
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