Wednesday, February 9, 2011
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.
# 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
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".
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.