Forced Writes?

Forced Writes are turned ON by default on Windows since the release of Firebird V1.0.

See the following item on the tracker: CORE-591

But Forced Writes were OFF on Linux even if they were turned ON, until an issue with Linux (Posix) was resolved in Firebird V2.1 Beta 2 and this fix has now also ben backported to Firebird V2.0.4.

See the following item on the tracker: CORE-1476

There was some discussion amongst the Firebird developers about the change (either to OFF on Win or to ON on Linux so we would have consistent defaults), but so far there has not been a final resolution since Windows and Linux handle I/O differently, and its perfectly valid to have different default settings. Before the fix in Firebird V2.1 Linux never did forced writes even when asked to, and pretty much everybody got used to the performance with Forced Writes off, when we turned them back on correctly the performance decrement was definitely going to be a problem.

Basically, Linux writes pages out of the page cache as often as possible, so the file on disk stays pretty much current, whilst Windows writes to disk only when it must, so the difference between the disk and cache pre Firebird V1.5 could be enormous - hours or days of changes if the database file was not closed correctly. So, although it may be preferable that all systems should run with forced writes on, for Firebird on Windows it was pretty critical.

Within Firebird V1.5 we attempted to make sure that the issue of Windows cache, the Firebird cache and what was on disk could not be vastly different even if Forced Writes were turn off by introducing the following two parameters in the Firebird configuration file. However the new parameters do not guarentee complete consistency between the cache and disk, for that you do need Forced Writes turned on.

The parameters:

# ----------------------------
#
# How often the pages are flushed on disk
# (for databases with ForcedWrites=Off only)
#
# Number of unflushed writes which will accumulate before they are
# flushed, at the next transaction commit. For non-Win32 ports,
# the default value is -1 (Disabled)
#
# Type: integer
#
#MaxUnflushedWrites = 100

#
# Number of seconds during which unflushed writes will accumulate
# before they are flushed, at the next transaction commit. For non-Win32
# ports, the default value is -1 (Disabled)
#
# Type: integer
#
#MaxUnflushedWriteTime = 5

Colissimo - What a joke.

If you want to send a package to anywhere in France, La Poste insist that you send it by Colissimo. Colissimo is more "expensive" than normal post, but it has insurance just in case its delivered late, goes missing or whatever you are sending gets broken.

Recently I sent a couple of Firebird mugs via Colissimo to a customer/Firebird user in France. After receipt and opening of the carefully packed box, the customer found that one of the mugs was cracked. After a brief email exchange he agreed to send me the damaged mug and I would send him a new one.

Once I had the broken mug, I asked the local post office for the basic repayment for the damaged mug from Colissimo. Their response was to hand me a form to fill in, and suggest I post the completed form direct to Colissimo. OK - job done - now I sit back and wait for them to send me the money for something they had managed to break.

After a couple of weeks a letter arrives telling me that they are looking into it, two weeks later another letter arrives, this one apologises, and says we cannot pay you the insurance because the reciever of the damaged goods should have filled in the complaint form and not the sender.

WTF? When you receive damaged goods from someone, what is the first thing you do?
Well, I would have thought, you contact the sender, ask if they want it back and ask for a replacement.... Its then the senders problem to deal with the people he used to send it.

Not in France it seems. By now we have paid for three lots of Colissimo (send original, send it back, send another) and wasted a whole lot of time. The result - huge profits for Colissimo, zero service for the customer. I now give up and throw the whole lot in the bin. Life is just too short to start aguing with a "fonctionnaire".

DEC founder Ken Olsen is dead - Silence at the Mill

The Register today contained an obituary to Ken Olsen, the man who created DEC. Reading the article brings back many memories for me. The first computer I worked with in the mid 1980s was a VAX 11/750, along with Dec Professionals. We implemented a distributed accounting system (MAS-M from Hoskyns) for BRS Northern. I then moved to BRS Group where we started writing a more advanced system using the latest DEC software at the time. ACMS, TDMS, RDB etc, I was the DBA responsible for designing and implementing the corporate database. Great times, great memories. I will raise a glass in your memory tonight.

A Quick Start Guide to Index Optimisation

Below a Quick Start Guide to index optimisation, courtesy of Pavel Cisar.

1. Always define ALL integrity constraints: primary, foreign and unique constraints. As this will automatically cover join optimisations.

2. Define a separate index on EACH column you will use for additional constraints in your queries (eg. WHERE clause filter conditions) that are not covered by point 1.

3. Update your index statistics regularly as you add/change/delete data in your database. A good general rule is to update statistics as part of your regular database maintenance routine (when you do a backup, sweep etc.), and/or whenever you add/change a quarter of all the rows, once the table is bigger than 10,000 rows.

4. Once you have a representative sample of real world data in your database, you can then evaluate the usability of the indices to eliminate those that will not help your queries (a worthless index will only slow down your inserts and updates!) and add new composite or expression indices that speed up specific queries.

5. Useless indices are typically those with low selectivity (few distinct values). Run some queries with filter condition on bad cardinality columns and check whether the optimizer uses the index or not. If the low selectivity index is always/often used with other conditions, you can improve it's selectivity and thus usability by
creating a composite index on columns that are used together for filter conditions instead of independent indices on these columns.

6. If you always/frequently use a group of columns for filter conditions, a composite index on these columns can boost the query performance, but do it only if you're not satisfied with performance that individual column indices provide.

Promenade - Chezelles - 2010

Every first sunday afternoon at 2.00pm in September (except for the year it was cancelled due to a clash with an agricultural show), there is a "promenade gastronomique" (a walk where you taste local produce) at Chezelles. The walk this Sunday was 9km (thankfully because the thermometer was showing 29 degress centigrade in the shade).

On the walk, we tasted local wines, goats cheese, jams, honey products and beer, and had a great time.

I have never seen this "walk" advertised, but its very popular. If you are in the area next year at the same time, then it might be worth having a go.

Cost this year was 4 Euros per person.

Stirling EH924 25th/26th July 1943 Essen

Its been a while since I posted anything to blog. There are no major reasons as to why I haven't posted, just haven't had anything major or interesting to say.

However while doing research about my Grandfather's RAF career I came across this comment in the 51 Squadron Operations Record book for the night of the 25th/26th July 1943.

Details of Sortie on Flight
.... "An aircraft flying immediately behind this Halifax was shot down in the Goch area. Halifax itself however returned to base without incident."

I was curious as to which aircraft was shot down, and a web search revealed the following on lostbombers.co.uk. The data on that site being taken from William Chorley's excellent books about RAF Bomber Command's losses during the Second World War. Note the information on lostbombers.co.uk is out of date and was compiled from William Chorley's books without permission from William or his publishers.

Stirling EH924 Information
Type Stirling
Serial Number EH924
Squadron 620
X1D QS-B
Operation Essen
Date 1 25th July 1943
Date 2 26th July 1943

Further Information

"Serial Range EH921 - EH961. 41 Stirling Mk.111. Part of a batch of 120 Short S.29 Stirling Mk.111. EH875-EH909; EH921-EH961;

EH977-EH996; EJ104-EJ127 Mk.111, of which EH897, EH950 and EJ106 were converted to Mk.1V. Delivered by Austin Motors Ltd between Jun43 and Sep43.
Contract No.B982939/39. Delivered to No.32MU 30Jun43, to No.620 Sqdn 11Jul43.
EH924 was one of three No.620 Sqdn Stirlings lost on this operation. See: BF511; EE906.
Airborne 2226 25Jul43 from Chedburgh. Cause of loss not established. Crashed at
Asperden, 3 km from Goch. Those killed were taken for burial in the Stadtfriedhof at Monchengladbach. They have been subsequently re-interred in the Rheinberg War Cemetery. Sgt J.D.Rathbone KIA Sgt J.H.Wallace KIA Sgt R.Wild KIA F/O J.F.Shepherd KIA Sgt J.F.Wells KIA Sgt A.Simons KIA Sgt D.H.Castling RCAF Inj Sgt D.H.Castling was confined to Hospital due injuries. No PoW No.

No other bomber lost that night crashed anywhere near Goch. So it looks as if EH924 was shot down by a German night-fighter. I hope somebody out there finds this information useful.

Dell Latitude D800

I have an "oldish" laptop, a Dell Latitude D800 to be precise which I have had for a few years. I use it when on the road, and my wife uses it at home. Recently I have had problems turning it on. This morning it refused to start compeletely, some extensive googling for a possible solution, turned up this nugget of information posted by PaxRiverWino on TechRepublic.

I have copied the information below - just in case it ever goes missing.

THIS FIXES IT. Dell Latitude 800 no power, no light, does not boot

Take out the battery and unplug from the AC adapter. Open your computer so you are facing the screen. There is a grey strip behind the keyboard that is about 1 inch on the left and a half inch on the right. It will say LATITUDE|D800 on it. Pry it open on both ends. On the right you will see a plug in device. Remove it from being seated and then reseat it. Replace the strip. I'll bet you that is all it takes.

PaxRiverWino - too right - This did indeed fix it!