On Fri, Feb 25, 2011 at 12:04 PM, Dan Kennedy <danielk1...@gmail.com> wrote: > On 02/26/2011 12:30 AM, Johns Daniel wrote: >> What is the reason was for having random filenames for the SQLite >> master journal file (like mine.db-mj501CA440, mine.db-mj1C17CCCC, >> mine.db-mj66677495, etc)? >> >> Here is the reason for this strange question. We have discovered that >> the JFFS2 filesystem in Linux is leaking kernel memory each time we >> open/close/delete a file with a different name! I have not figured out >> how to fix the leak in JFFS2 yet. I was wondering whether this >> workaround is safe: Change our SQLite code to always use a fixed >> filename (like mine.db-mjMASTER). > > The problem is that if a crash occurs during a multi-file commit, > the master journal cannot be removed from the file-system until > the transactions have been rolled back for all database files. > And this doesn't happen until the first time each of them is > read. > > For example, say you have two databases in a transaction - mine.db > and yours.db. A crash happens while committing the transaction > and the file mine.db-mjMASTER is left in the file-system. > > Following recovery, if a process tries to read mine.db, the > transaction on mine.db will be rolled back. But the master > journal file cannot be deleted yet (since the interrupted > transaction on yours.db has not yet been reverted). > > If the process then attaches "ours.db" and then tries to > write a multi-file transaction that involves mine.db and > ours.db it has a problem - it wants to create and populate > mine.db-mjMASTER but it cannot, since that file is still > in use. > > We use the random names to get around this problem. > You could probably get away with using "-mjMASTER1", > "-mjMASTER2" etc. if you wanted to. See the loop around > line 1729 of vdbeaux.c. >
Thank you very much for this info, Dan! Very useful. >From your description, it sounds like this requirement only applies if there are 3 or more databases. Is this an issue with 2 databases? We have 2 databases. We open one and attach the other one to it at the very beginning of our single (multi-threaded) database process. Subsequently, we starting issuing SQL statements that could affect either one or both databases. It seems we could get by with a single master-journal filename. Do you agree? -- Johns _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
