> From: Ross Walker [mailto:rswwal...@gmail.com] > Sent: Friday, June 17, 2011 9:48 PM > > The on-disk buffer is there so data is ready when the hard drive head lands, > without it the drive's average rotational latency will trend higher due to > missed landings because the data wasn't in buffer at the right time. > > The read buffer is to allow the disk to continuously read sectors whether the > system bus is ready to transfer or not. Without it, sequential reads wouldn't > last long enough to reach max throughput before they would have to pause > because of bus contention and then suffer a rotation of latency hit which > would kill read performance.
And it turns out ... Ross is the winner. ;-) My hypothesis wasn't right, and whoever said a single disk would hog the bus in an idle state, that's also wrong. Conclusion: Yes it matters to enable the write_cache. But the reason it matters is to ensure the right data is present at the right time. NOT because of any idle bus blocking. Here's the test: I tested writing to a bunch (4) of disks simultaneously at maximum throughput, with the write_cache enabled. This is on a 6Gbit bus, they all performed 1.0 Gbit/sec which was precisely the mfgr spec. Then I disabled write_cache on all the disks and repeated the test. They all dropped to 750 Mbit/sec. If the idle bus contention were correct, then the total bus speed would have been limited to the max throughput of a single disk (1Gbit). But I was easily able to sustain 3Gbit, thus disproving the idle bus contention. If the filesystem write buffer were making the disk write_cache irrelevant, as I conjectured, then the total throughput would have been the same, regardless of whether the write_cache was enabled or disabled. Since performance dropped with write_cache disabled, it disproves my hypothesis. No further testing was necessary. I'm not interested in how much performance difference there is - or under which specific conditions they occur. I am only interested in the existence of a performance difference. So the conclusion is yes, you want to enable your disk write cache (assuming all the data on your disk is managed by ZFS.) _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
