Hi, david@xxxxxxx wrote: > On Sat, 10 Jan 2009, Gregory Stark wrote: >> I think the idea is that with SSDs or a RAID with a battery backed >> cache you >> can leave fsync on and not have any significant performance hit since >> the seek >> times are very fast for SSD. They have limited bandwidth but bandwidth >> to the >> WAL is rarely an issue -- just latency. That's also my understanding. > with SSDs having extremely good read speeds, but poor (at least by > comparison) write speeds I wonder if any of the RAID controllers are > going to get a mode where they cache writes, but don't cache reads, > leaving all ofyour cache to handle writes. My understanding of SSDs so far is, that they are not that bad at writing *on average*, but to perform wear-leveling, they sometimes have to shuffle around multiple blocks at once. So there are pretty awful spikes for writing latency (IIRC more than 100ms has been measured on cheaper disks). A battery backed cache could theoretically flatten those, as long as your avg. WAL throughput is below the SSDs avg. writing throughput. Regards Markus Wanner -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
