2014-03-17 21:12 GMT+09:00 Fujii Masao <masao.fu...@gmail.com>: > On Mon, Mar 17, 2014 at 10:20 AM, Robert Haas <robertmh...@gmail.com> > wrote: > > On Sun, Mar 16, 2014 at 6:23 AM, MauMau <maumau...@gmail.com> wrote: > >> The PostgreSQL documentation describes cp (on UNIX/Linux) or copy (on > >> Windows) as an example for archive_command. However, cp/copy does not > sync > >> the copied data to disk. As a result, the completed WAL segments would > be > >> lost in the following sequence: > >> > >> 1. A WAL segment fills up. > >> > >> 2. The archiver process archives the just filled WAL segment using > >> archive_command. That is, cp/copy reads the WAL segment file from > pg_xlog/ > >> and writes to the archive area. At this point, the WAL file is not > >> persisted to the archive area yet, because cp/copy doesn't sync the > writes. > >> > >> 3. The checkpoint processing removes the WAL segment file from pg_xlog/. > >> > >> 4. The OS crashes. The filled WAL segment doesn't exist anywhere any > more. > >> > >> Considering the "reliable" image of PostgreSQL and widespread use in > >> enterprise systems, I think something should be done. Could you give me > >> your opinions on the right direction? Although the doc certainly > escapes by > >> saying "(This is an example, not a recommendation, and might not work > on all > >> platforms.)", it seems from pgsql-xxx MLs that many people are following > >> this example. > >> > >> * Improve the example in the documentation. > >> But what command can we use to reliably sync just one file? > >> > >> * Provide some command, say pg_copy, which copies a file synchronously > by > >> using fsync(), and describes in the doc something like "for simple use > >> cases, you can use pg_copy as the standard reliable copy command." > > > > +1. This won't obviate the need for tools to manage replication, but > > it would make it possible to get the simplest case right without > > guessing. > > +1, too. > > And, what about making pg_copy call posix_fadvise(DONT_NEED) against the > archived file after the copy? Also It might be good idea to support the > direct > copy of the file to avoid wasting the file cache.
Use direct_cp. http://directcp.sourceforge.net/direct_cp.html Regards, -- Mitsumasa KONDO NTT Open Source Software Center
