Michael, it's an interesting hypothesis. Maybe you can describe the following results. Recently I made a test program that just reads a file sequentially with different block sizes and results are still confusing. This was the same SSD drive and the functions were basically from api: CreateFile/ReadFile
128, 256: 5MB/Sec 512: 20MB/Sec 1024: 25MB/Sec. 2048: 41MB/Sec 4096 - 65536: ~50MB/Sec. Strangely I didn't see such a big difference between slowest and fastest scenario (if we exclude the sizes below 1024), as you see the difference is only x2. I have only one explanation looking at the numbers. Although it's well-known that sqlite reads only full pages, if it sometimes does partial reading, this 5MB/Sec drop for <256 reading can affect linear speed of 25 MB/Sec to end up as 12MB/Sec. But it's just a guess. Max On Mon, Feb 28, 2011 at 4:43 PM, Black, Michael (IS) <michael.bla...@ngc.com > wrote: > Those numbers make sense to me. > > Since count is doing a linear walk through....correct? > > #1 SSDs more than likely don't do "read ahead" like a disk drive. So what > you're seeing is what read-ahead does for you. > #2 Count is doing a linear scan of the table...Probability of 2K containing > the next page after 1K -- 100% (2X performance improvment) > #3 Probability of 4K containing the next page after 2K -- 100% (2X > improvement). > #4 Probability of 8K containing the next page after 4K -- here the > probability either drops or we're starting to hit the bandwidth of > SSD+Sqlite -- I'm tempted to say that it's not the probability that drops. > > ________________________________________ > From: sqlite-users-boun...@sqlite.org [sqlite-users-boun...@sqlite.org] on > behalf of Max Vlasov [max.vla...@gmail.com] > > my timing for Select Count for 400-500MB Bases and the table with about > 1,000,000 records > > 1024: > 40 Seconds, 98% system+user time, 500 MB Read, 12 MB/Sec > 2048: > 22 Seconds, 94% system+user time, 500 MB Read, 20 MB/Sec > 4096 (This NTFS Cluster size) > 11 Seconds, 96% system+user time, 450 MB Read, 32 MB/Sec > 8192 > 8 Seconds, 87% system+user time, 420 MB Read, 50 MB/Sec > 32768 > 8 Seconds, 56% system+user time, 410 MB Read, 50 MB/Sec > > Max > _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
