BackUp Gmail 1.9.4

Trusted Mac download BackUp Gmail 1.9.4. Virus-free and 100% clean download. Get BackUp Gmail alternative downloads.

On Wed, Jun 3, 2015 at 4:48 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Wed, Jun 3, 2015 at 3:42 PM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>> Thomas Munro wrote:
>>> On Tue, Jun 2, 2015 at 9:30 AM, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>>> > My guess is that the file existed, and perhaps had one or more pages,
>>> > but the wanted page doesn't exist, so we tried to read but got 0 bytes
>>> > back. read() returns 0 in this case but doesn't set errno.
>>> >
>>> > I didn't find a way to set things so that the file exists but is of
>>> > shorter contents than oldestMulti by the time the checkpoint record is
>>> > replayed.
>>>
>>> I'm just starting to learn about the recovery machinery, so forgive me
>>> if I'm missing something basic here, but I just don't get this. As I
>>> understand it, offsets/0046 should either have been copied with that
>>> page present in it if it existed before the backup started (apparently
>>> not in this case), or extended to contain it by WAL records that come
>>> after the backup label but before the checkpoint record that
>>> references it (also apparently not in this case).
>>
>> Exactly --- that's the spot at which I am, also. I have had this
>> spinning in my head for three days now, and tried every single variation
>> that I could think of, but like you I was unable to reproduce the issue.
>> However, our customer took a second base backup and it failed in exactly
>> the same way, module some changes to the counters (the file that
>> didn't exist was 004B rather than 0046). I'm still at a loss at what
>> the failure mode is. We must be missing some crucial detail ...
>
> I have finally reproduced that error! See attached repro shell script.
>
> The conditions are:
>
> 1. next multixact oldest multixact (no active multixacts, pointing
> past the end)
> 2. next multixact would be the first item on a new page (multixact % 2048 0)
> 3. the page must not be the first in a segment (or we'd get the
> read-zeroes case)
>
> That gives you odds of 1/2048 * 31/32 * (probability of a wraparound
> vacuum followed by no multixact creations right before your backup
> checkpoint). That seems like reasonably low odds... if it happened
> twice in a row, maybe I'm missing something here and there is some
> other way to get this...
>
> I realise now that this is actually a symptom of a problem spotted by
> Noah recently:
>
> http://www.postgresql.org/message-id/20150601045534.GB23587@tornado.leadboat.com
>
> He noticed the problem for segment boundaries, when not in recovery.
> In recovery, segment boundaries don't raise an error (the read-zeroes
> case applies), but page boundaries do. The fix is probably to do
> nothing if they are the same, as we do elsewhere, like in the attached
> patch.

Backup Gmail 1.9.4 Free

Actually, we still need to call DetermineSafeOldestOffset in that
case. Otherwise, if someone goes from lots of multixacts to none, the
stop point won't advance.

How To Backup Gmail

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company