Jon Nelson wrote:
One thing to note is that where on a disk things sit can make a /huge/ difference - depending on if Ubuntu is /here/ and RHEL is /there/ and so on can make a factor of 2 or more difference. The outside tracks of most modern SATA disks can do around 120MB/s. The inside tracks aren't even half of that.
You're talking about changes in sequential read and write speed due to Zone Bit Recording (ZBR) AKA Zone Constant Angular Velocity (ZCAV). What I was measuring was commit latency time on small writes. That doesn't change as you move around the disk, since it's tied to the raw rotation speed of the drive rather than density of storage in any zone. If I get to something that's impacted by sequential transfers rather than rotation time, I'll be sure to use the same section of disk for that. It wasn't really necessary to get these initial gross numbers anyway. What I was looking for is the about 10:1 speedup seen on this hardware when the write cache is used, which could easily be seen even were there ZBR differences involved.
-- Greg Smith 2ndQuadrant US greg@xxxxxxxxxxxxxxx Baltimore, MD PostgreSQL Training, Services and Support www.2ndQuadrant.us "PostgreSQL 9.0 High Performance": http://www.2ndQuadrant.com/books -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
