On Thu, Nov 6, 2008 at 4:03 PM, Scott Marlowe <scott.marlowe@xxxxxxxxx> wrote: > On Thu, Nov 6, 2008 at 4:04 PM, Kevin Grittner <Kevin.Grittner@xxxxxxxxxxxx> wrote: >> "Scott Marlowe" <scott.marlowe@xxxxxxxxx> wrote: >>> Without write barriers in my file system an fsync request will >>> be immediately returned true, correct? >> >> Not as I understand it; although it will be pretty fast if it all fits >> into the battery backed cache. > > OK, thought exercise time. There's a limited size for the cache. > Let's assume it's much smaller, say 16Megabytes. We turn off write > barriers. We start writing data to the RAID array faster than the > disks can write it. At some point, the data flowing into the cache is > backing up into the OS. Without write barriers, the second we call an > fsync it returns true. But the data's not in the cache yet, or on the > disk. Machine crashes, data is incoherent. > > But that's assuming write barriers work as I understand them. Let's try to clear up a couple things: 1. We are talking about 3 different memory caches in order from low to high: Disk cache, Controller cache (BBU in this case) and OS cache. 2. A write barrier instructs the lower level hardware that commands issued before the barrier must be written to disk before commands issued after the barrier. Write barriers are used to ensure that data written to disk is written in such a way as to maintain filesystem consistency, without losing all the benefits of a write cache. 3. A fsync call forces data to be synced to the controller. This means that whenever you call fsync, at the very minimum, the data will have made it to the controller. How much further down the line will depend on whether or not the controller is in WriteBack or WriteThrough mode and whether or not the disk is also caching writes. So in your example, if the OS is caching some writes and fsync is called, it won't be returned until at a minimum the controller has accepted all the data, regardless of whether or not write barriers are enabled. In theory, it should be safe to disable write barriers if you have a BBU because the BBU should guarantee that all writes will eventually make it to disk (or at least reduce the risk of that not happening to an acceptable level). -Dave -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
