Re: Losing data

On Thu, 2008-06-19 at 16:55 +0100, Garry Saddington wrote:
> I have had a serious loss of data and wondered if anyone could shed any light
> on what may have happened.
> My users have been writing reports on students. No error messages have been
> produced and when called back up the reports seem to be present at the time
> of writing. However, next day they have disappeared, and they do not appear
> in a pg_dump. They seem to have been kept in memory and never written to
> disk.
> We are using Zope and connecting to Postgres through psycopg on Centos 5.
> I suspect a hard disk failure but any other ideas would be welcome.
> Would these reports be in the WAL?

If it was hardware related you would know, quickly. This sounds a great
deal more like an application level interaction. Perhaps your zope
application caches things for a while before committing to disk?

Are you familiar with the actual database itself? Have you checked the
database logs for errors?

Sincerely,

Joshua D. rae

I have had a serious loss of data and wondered if anyone could shed any light
on what may have happened.
My users have been writing reports on students. No error messages have been
produced and when called back up the reports seem to be present at the time
of writing. However, next day they have disappeared, and they do not appear
in a pg_dump. They seem to have been kept in memory and never written to
disk.
We are using Zope and connecting to Postgres through psycopg on Centos 5.
I suspect a hard disk failure but any other ideas would be welcome.
Would these reports be in the WAL?
regards
garry

On Thursday 19 June 2008 16:55, Joshua D. Drake wrote:
> On Thu, 2008-06-19 at 16:55 +0100, Garry Saddington wrote:
> > I have had a serious loss of data and wondered if anyone could shed any
> > light on what may have happened.
> > My users have been writing reports on students. No error messages have
> > been produced and when called back up the reports seem to be present at
> > the time of writing. However, next day they have disappeared, and they do
> > not appear in a pg_dump. They seem to have been kept in memory and never
> > written to disk.
> > We are using Zope and connecting to Postgres through psycopg on Centos 5.
> > I suspect a hard disk failure but any other ideas would be welcome.
> > Would these reports be in the WAL?
>
> If it was hardware related you would know, quickly. This sounds a great
> deal more like an application level interaction. Perhaps your zope
> application caches things for a while before committing to disk?
Yes I thought of this but once the report is sent to the DB a separate query
is run to get all of that teacher's reports and these are then displayed on a
new page. They all appear here but then disappear later. Zope has transaction
machinery that rolls everything back on an error, so Postgres must have
indicated a successful write somehow. I read in a Postgres manual that the
hard disk may report to the OS that a write has occured when it actually has
not, is this possible? Oh, and the problem has been intermittant. Another
thing that happened this morning is that Postgres had today as 18/06/2008
when in fact it was 19/06/2008 and the OS reported this correctly. Restarting
postgres sorted it, could this be the problem?
Regards
Garry

On Thu, Jun 19, 2008 at 9:55 AM, Garry Saddington
<garry(at)schoolteachers(dot)co(dot)uk> wrote:
> I have had a serious loss of data and wondered if anyone could shed any light
> on what may have happened.
> My users have been writing reports on students. No error messages have been
> produced and when called back up the reports seem to be present at the time
> of writing. However, next day they have disappeared, and they do not appear
> in a pg_dump. They seem to have been kept in memory and never written to
> disk.
> We are using Zope and connecting to Postgres through psycopg on Centos 5.
> I suspect a hard disk failure but any other ideas would be welcome.
> Would these reports be in the WAL?
> regards

Just a guess, but do you have some kind of search engine pointing at
the application? Could it be hitting a "delete" link maybe?

In response to Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>:

> On Thursday 19 June 2008 16:55, Joshua D. Drake wrote:
> > On Thu, 2008-06-19 at 16:55 +0100, Garry Saddington wrote:
> > > I have had a serious loss of data and wondered if anyone could shed any
> > > light on what may have happened.
> > > My users have been writing reports on students. No error messages have
> > > been produced and when called back up the reports seem to be present at
> > > the time of writing. However, next day they have disappeared, and they do
> > > not appear in a pg_dump. They seem to have been kept in memory and never
> > > written to disk.
> > > We are using Zope and connecting to Postgres through psycopg on Centos 5.
> > > I suspect a hard disk failure but any other ideas would be welcome.
> > > Would these reports be in the WAL?
> >
> > If it was hardware related you would know, quickly. This sounds a great
> > deal more like an application level interaction. Perhaps your zope
> > application caches things for a while before committing to disk?
> Yes I thought of this but once the report is sent to the DB a separate query
> is run to get all of that teacher's reports and these are then displayed on a
> new page. They all appear here but then disappear later. Zope has transaction
> machinery that rolls everything back on an error, so Postgres must have
> indicated a successful write somehow. I read in a Postgres manual that the
> hard disk may report to the OS that a write has occured when it actually has
> not, is this possible?

No. If that happens you end up with corrupt disks. The chance of that
going unnoticed by the OS is pretty slim.

> Oh, and the problem has been intermittant. Another
> thing that happened this morning is that Postgres had today as 18/06/2008
> when in fact it was 19/06/2008 and the OS reported this correctly. Restarting
> postgres sorted it, could this be the problem?

Sounds to me like there's something seriously wrong with you OS or your
PostgreSQL install. What version of PostgreSQL is this? What OS?

--
Bill Moran
Collaborative Fusion Inc.
http://people.collaborativefusion.com/~wmoran/

wmoran(at)collaborativefusion(dot)com
Phone: 412-422-3463x4023

On Thursday 19 June 2008, Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>
wrote:
> I read in a
> Postgres manual that the hard disk may report to the OS that a write has
> occured when it actually has not, is this possible?

Yeah. But unless the power suddenly turned off that wouldn't cause data
loss.

> Oh, and the problem
> has been intermittant. Another thing that happened this morning is that
> Postgres had today as 18/06/2008 when in fact it was 19/06/2008 and the
> OS reported this correctly. Restarting postgres sorted it, could this be
> the problem?

I strongly suspect the problem is between the keyboard and the chair.

In any case, however, if PostgreSQL reported the transaction complete and
the machine didn't experience any hardware problems (like sudden power or
disk failure), I would certainly not suspect PostgreSQL as the source of
the problem.

--
Alan

Garry Saddington wrote:
> On Thursday 19 June 2008 18:15, Alan Hodgson wrote:
>> On Thursday 19 June 2008, Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>
>>
>> wrote:
>>> I read in a
>>> Postgres manual that the hard disk may report to the OS that a write has
>>> occured when it actually has not, is this possible?
>> Yeah. But unless the power suddenly turned off that wouldn't cause data
>> loss.
>>
>>> Oh, and the problem
>>> has been intermittant. Another thing that happened this morning is that
>>> Postgres had today as 18/06/2008 when in fact it was 19/06/2008 and the
>>> OS reported this correctly. Restarting postgres sorted it, could this be
>>> the problem?
>> I strongly suspect the problem is between the keyboard and the chair.
> I'd love to agree, but I have seen this first hand as a user!
>> In any case, however, if PostgreSQL reported the transaction complete and
>> the machine didn't experience any hardware problems (like sudden power or
>> disk failure), I would certainly not suspect PostgreSQL as the source of
>> the problem.
> What has happened to the reports then? I have used this combination of Zope
> and Postgres for 5 years with no problems like this before and we have
> written one complete set of reports on this server in the past 6 weeks. The
> problem seems to have started last friday, when reports started to go
> missing.

What has changed prior to Friday?

--
Until later, Geoffrey

Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety.
- Benjamin Franklin

On Thursday 19 June 2008 18:10, Bill Moran wrote:
> In response to Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>:
> > On Thursday 19 June 2008 16:55, Joshua D. Drake wrote:
> > > On Thu, 2008-06-19 at 16:55 +0100, Garry Saddington wrote:
> > > > I have had a serious loss of data and wondered if anyone could shed
> > > > any light on what may have happened.
> > > > My users have been writing reports on students. No error messages
> > > > have been produced and when called back up the reports seem to be
> > > > present at the time of writing. However, next day they have
> > > > disappeared, and they do not appear in a pg_dump. They seem to have
> > > > been kept in memory and never written to disk.
> > > > We are using Zope and connecting to Postgres through psycopg on
> > > > Centos 5. I suspect a hard disk failure but any other ideas would be
> > > > welcome. Would these reports be in the WAL?
> > >
> > > If it was hardware related you would know, quickly. This sounds a great
> > > deal more like an application level interaction. Perhaps your zope
> > > application caches things for a while before committing to disk?
> >
> > Yes I thought of this but once the report is sent to the DB a separate
> > query is run to get all of that teacher's reports and these are then
> > displayed on a new page. They all appear here but then disappear later.
> > Zope has transaction machinery that rolls everything back on an error, so
> > Postgres must have indicated a successful write somehow. I read in a
> > Postgres manual that the hard disk may report to the OS that a write has
> > occured when it actually has not, is this possible?
>
> No. If that happens you end up with corrupt disks. The chance of that
> going unnoticed by the OS is pretty slim.
>
> > Oh, and the problem has been intermittant. Another
> > thing that happened this morning is that Postgres had today as 18/06/2008
> > when in fact it was 19/06/2008 and the OS reported this correctly.
> > Restarting postgres sorted it, could this be the problem?
>
> Sounds to me like there's something seriously wrong with you OS or your
> PostgreSQL install. What version of PostgreSQL is this? What OS?
>
> --
Centos 5 with the Posgres that comes with it - 8.1
regards
garry

On Thu, 2008-06-19 at 19:06 +0100, Garry Saddington wrote:
> On Thursday 19 June 2008 18:15, Alan Hodgson wrote:
> > On Thursday 19 June 2008, Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>

Although I appreciate that this is a funky problem, the problem doesn't
yet exist and we are operating in a diagnostic vacuum.

Can you duplicate the problem now?

Do you have any logs from PostgreSQL that describe the session that
either previously or currently (based on your tests) the problem?

Do those logs actually show ERROR?

What about your apachelog or zope logs?

Have you turned up the logging in all three components to ensure that
the problem actually does exist?

Sincerely,

Joshua D. Drake

> regards
> Garry
>

On Thursday 19 June 2008 18:15, Alan Hodgson wrote:
> On Thursday 19 June 2008, Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>
>
> wrote:
> > I read in a
> > Postgres manual that the hard disk may report to the OS that a write has
> > occured when it actually has not, is this possible?
>
> Yeah. But unless the power suddenly turned off that wouldn't cause data
> loss.
>
> > Oh, and the problem
> > has been intermittant. Another thing that happened this morning is that
> > Postgres had today as 18/06/2008 when in fact it was 19/06/2008 and the
> > OS reported this correctly. Restarting postgres sorted it, could this be
> > the problem?
>
> I strongly suspect the problem is between the keyboard and the chair.
I'd love to agree, but I have seen this first hand as a user!
>
> In any case, however, if PostgreSQL reported the transaction complete and
> the machine didn't experience any hardware problems (like sudden power or
> disk failure), I would certainly not suspect PostgreSQL as the source of
> the problem.
What has happened to the reports then? I have used this combination of Zope
and Postgres for 5 years with no problems like this before and we have
written one complete set of reports on this server in the past 6 weeks. The
problem seems to have started last friday, when reports started to go
missing.
regards
Garry

On Thursday 19 June 2008 18:09, Scott Marlowe wrote:
> On Thu, Jun 19, 2008 at 9:55 AM, Garry Saddington
>
> <garry(at)schoolteachers(dot)co(dot)uk> wrote:
> > I have had a serious loss of data and wondered if anyone could shed any
> > light on what may have happened.
> > My users have been writing reports on students. No error messages have
> > been produced and when called back up the reports seem to be present at
> > the time of writing. However, next day they have disappeared, and they do
> > not appear in a pg_dump. They seem to have been kept in memory and never
> > written to disk.
> > We are using Zope and connecting to Postgres through psycopg on Centos 5.
> > I suspect a hard disk failure but any other ideas would be welcome.
> > Would these reports be in the WAL?
> > regards
>
> Just a guess, but do you have some kind of search engine pointing at
> the application? Could it be hitting a "delete" link maybe?
Don't think so, once a teacher has entered a report there is no way that they
can delete it.
Regards
Garry

Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk> writes:
> Oh, and the problem has been intermittant. Another
> thing that happened this morning is that Postgres had today as 18/06/2008
> when in fact it was 19/06/2008 and the OS reported this correctly.

Two theories about that one:

1. Postgres' timezone setting is far away from where you are.

2. You were in a transaction that had been open since yesterday.
now() and related functions show time of transaction start.

#2 is interesting since forgetting to commit an open transaction might
also explain the "data loss" complaint.

regards, tom lane

On Thursday 19 June 2008 19:03, Joshua D. Drake wrote:
> On Thu, 2008-06-19 at 19:06 +0100, Garry Saddington wrote:
> > On Thursday 19 June 2008 18:15, Alan Hodgson wrote:
> > > On Thursday 19 June 2008, Garry Saddington <garry(at)schoolteachers(dot)co(dot)uk>
>
> Although I appreciate that this is a funky problem, the problem doesn't
> yet exist and we are operating in a diagnostic vacuum.
>
> Can you duplicate the problem now?
The problem is not consistent, some reports are lsot others not.
>
> Do you have any logs from PostgreSQL that describe the session that
> either previously or currently (based on your tests) the problem?
These are at school so I will look tomorrow

>
> Do those logs actually show ERROR?
There are no errors relating to reports
>
> What about your apachelog or zope logs?
Again tomorrow
>
> Have you turned up the logging in all three components to ensure that
> the problem actually does exist?
Will do this tomorrow
regards and thanks for your efforts.

PS
Could it be somethiing to do with libpq.so.4/5 required by psycopg and will
the WAL hold data to be replayed?

On Thu, 2008-06-19 at 20:29 +0100, Garry Saddington wrote:
> On Thursday 19 June 2008 19:03, Joshua D. Drake wrote:
> > On Thu, 2008-06-19 at 19:06 +0100, Garry Saddington wrote:
> > > On Thursday 19 June 2008 18:15, Alan Hodgson wrote:
> > > > On Thursday 19 June 2008, Garry Saddington
> <garry(at)schoolteachers(dot)co(dot)uk>
> >
> PS
> Could it be somethiing to do with libpq.so.4/5 required by psycopg and will

I think it certainly has something to do with psycopg and the way it
handles commits. Especially based on the description of what you say
above where some work and some don't. My guess (as I mentioned
previously) is there is a bit of code in your report generator that
doesn't call commit correctly.

> the WAL hold data to be replayed?

Only if you are pushing the logs off for standby operations and only if
you commit.

Sincerely,

Joshua D. Drake

>

Hi,

Garry Saddington wrote:
> On Thursday 19 June 2008 16:55, Joshua D. Drake wrote:
>> On Thu, 2008-06-19 at 16:55 +0100, Garry Saddington wrote:
...

> Yes I thought of this but once the report is sent to the DB a separate query
> is run to get all of that teacher's reports and these are then displayed on a
> new page. They all appear here but then disappear later. Zope has transaction
> machinery that rolls everything back on an error, so Postgres must have

are you running the report within the same request that made the write?
In this case you would be in the same transaction (and see uncommitted
data) if then an exeption is raised after you display the report
the transaction would be rolled back. You should check your zope logs.

Regards
Tino

On Thursday 19 June 2008 14:06:38 Garry Saddington wrote:
> > In any case, however, if PostgreSQL reported the transaction complete and
> > the machine didn't experience any hardware problems (like sudden power or
> > disk failure), I would certainly not suspect PostgreSQL as the source of
> > the problem.
>
> What has happened to the reports then? I have used this combination of Zope
> and Postgres for 5 years with no problems like this before and we have
> written one complete set of reports on this server in the past 6 weeks. The
> problem seems to have started last friday, when reports started to go
> missing.

Out of curiosity, what is your vacuum strategy?

--
Robert Treat
Build A Brighter LAMP :: Linux Apache {middleware} PostgreSQL

Robert Treat <xzilla(at)users(dot)sourceforge(dot)net> writes:
> On Thursday 19 June 2008 14:06:38 Garry Saddington wrote:
>> The problem seems to have started last friday, when reports started to go
>> missing.

> Out of curiosity, what is your vacuum strategy?

If you're thinking "transaction ID wraparound", I believe we can rule
that out, because Garry says he's running PG 8.1. 8.1 is not terribly
proactive about preventing wraparound (no forced autovacuums), but it
will squawk loudly about impending wraparound and shut down before it
hits the wall.

My bet is that something was changed in the client-side software last
week that has broken its commit-issuing behavior.

regards, tom lane

On Friday 20 June 2008 05:26, Robert Treat wrote:
> On Thursday 19 June 2008 14:06:38 Garry Saddington wrote:
> > > In any case, however, if PostgreSQL reported the transaction complete
> > > and the machine didn't experience any hardware problems (like sudden
> > > power or disk failure), I would certainly not suspect PostgreSQL as the
> > > source of the problem.
> >
> > What has happened to the reports then? I have used this combination of
> > Zope and Postgres for 5 years with no problems like this before and we
> > have written one complete set of reports on this server in the past 6
> > weeks. The problem seems to have started last friday, when reports
> > started to go missing.
>
> Out of curiosity, what is your vacuum strategy?
We back up and vacuum at the same time each day.
garry