Re: [PATCH] dump: make dump take Xapian write lock

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+From: Mark Walters <markwalters1009@gmail.com>\r
+To: David Bremner <david@tethera.net>, notmuch@notmuchmail.org\r
+Subject: Re: [PATCH] dump: make dump take Xapian write lock\r
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+\r
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+On Tue, 15 Jul 2014, David Bremner <david@tethera.net> wrote:\r
+> Mark Walters <markwalters1009@gmail.com> writes:\r
+>\r
+>>\r
+>> Discussion with Olly on irc indicates that this is currently the best\r
+>> solution: in xapian trunk there may be better possibilities using\r
+>> snapshots but they need to make it to a release and propogate out to\r
+>> users before we can switch approach.\r
+>\r
+> I agree that this seems to be the only feasible approach to make dump\r
+> atomic.  I'm a little unsure about the benefits to the end user though.\r
+> Currently, the user has to check the return code of dump to ensure it\r
+> completed correctly.  With this change, the user will still have to\r
+> check that dump did not error out when trying to acquire a write lock.\r
+> The following ticket\r
+>\r
+>       http://trac.xapian.org/ticket/275\r
+>\r
+> suggests that if we wanted to a blocking open, we'd be on our own.\r
+>\r
+> Did I miss something here about the benefits? I agree that failing\r
+> earlier is nicer, but is that it?\r
+\r
+You are correct that we may still fail. I think there are two\r
+advantages in addition to failing earlier. \r
+\r
+First, I think that if the dump fails there will not be any output file:\r
+I think the old version would leave a partial dump (note I haven't\r
+checked that carefully). This makes it more likely the failing dump is\r
+noticed.\r
+\r
+Secondly, this version will only fail if the database is being written\r
+to when it starts: the old version would fail if something attempted to\r
+write to it at any point during the dump, which can be relatively\r
+lengthy. This should at least reduce the frequency of problems.\r
+\r
+Best wishes\r
+\r
+Mark\r
+\r