Обсуждение: Re: [HACKERS] fsync method checking
On 25 Mar, Manfred Spraul wrote: > Tom Lane wrote: > >>markw@osdl.org writes: >> >> >>>I could certainly do some testing if you want to see how DBT-2 does. >>>Just tell me what to do. ;) >>> >>> >> >>Just do some runs that are identical except for the wal_sync_method >>setting. Note that this should not have any impact on SELECT >>performance, only insert/update/delete performance. >> >> > I've made a test run that compares fsync and fdatasync: The performance > was identical: > - with fdatasync: > > http://khack.osdl.org/stp/290607/ > > - with fsync: > http://khack.osdl.org/stp/290483/ > > I don't understand why. Mark - is there a battery backed write cache in > the raid controller, or something similar that might skew the results? > The test generates quite a lot of wal traffic - around 1.5 MB/sec. > Perhaps the writes are so large that the added overhead of syncing the > inode is not noticable? > Is the pg_xlog directory on a seperate drive? > > Btw, it's possible to request such tests through the web-interface, see > http://www.osdl.org/lab_activities/kernel_testing/stp/script_param.html We have 2 Adaptec 2200s controllers, without the battery backed add-on, connected to four 10-disk arrays in those systems. I can't think of anything off hand that would skew the results. The pg_xlog directory is not on a separate drive. I haven't found the best way to lay out of the drives on those systems yet, so I just have everything on a 28 drive lvm2 volume. Mark
markw@osdl.org wrote: > > I've made a test run that compares fsync and fdatasync: The performance > > was identical: > > - with fdatasync: > > > > http://khack.osdl.org/stp/290607/ > > > > - with fsync: > > http://khack.osdl.org/stp/290483/ > > > > I don't understand why. Mark - is there a battery backed write cache in > > the raid controller, or something similar that might skew the results? > > The test generates quite a lot of wal traffic - around 1.5 MB/sec. > > Perhaps the writes are so large that the added overhead of syncing the > > inode is not noticable? > > Is the pg_xlog directory on a seperate drive? > > > > Btw, it's possible to request such tests through the web-interface, see > > http://www.osdl.org/lab_activities/kernel_testing/stp/script_param.html > > We have 2 Adaptec 2200s controllers, without the battery backed add-on, > connected to four 10-disk arrays in those systems. I can't think of > anything off hand that would skew the results. > > The pg_xlog directory is not on a separate drive. I haven't found the > best way to lay out of the drives on those systems yet, so I just have > everything on a 28 drive lvm2 volume. We don't actually extend the WAL file during writes (preallocated), and the access/modification timestamp is only in seconds, so I wonder of the OS only updates the inode once a second. What else would change in the inode more frequently than once a second? -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073
Bruce, > We don't actually extend the WAL file during writes (preallocated), and > the access/modification timestamp is only in seconds, so I wonder of the > OS only updates the inode once a second. What else would change in the > inode more frequently than once a second? What about really big writes, when WAL files are getting added/recycled? -- -Josh Berkus Aglio Database Solutions San Francisco