[DB-SIG] Q: database integrity, flush and corrupted files....

[M.-A. Lemburg]

Michael Scharf wrote:
> Hi,
> 
> I am writing a little disk based hashset (like bsddb or gdbm).

You do know about mxBeeBase, do you ?

> I am trying to find info about information about what
> happens in the worst case if the application is killed in
> the wrong moment (e.g. in the middle of a flush). In
> particular: if I do:
> 
>   f.write(x)
>   f.write(y)
> 
> can I assume that when y gets flushed that x has been flushed?

No; flushing has to be done explicitly using f.flush() for this.

> If I do:
> 
>   pos=f.tell()
>   f.write(x)
>   f.seek(pos-len(x)-len(y)) # going before x
>   f.write(y)
> 
> Do I have to expect that y might be flushed before x?
> 
> Or is auto-flushing completely random (I am talking
> about windows)?

Flushing usually refers to writing line buffers out to the
disk. Whether the file system does write-through or not
depends on the settings of the file system. It also depends
on how you open the file, ie. buffered or not.

> Does anybody know a pointer where I can read about this?

man fflush and the Python docs on files and the os module.

-- 
Marc-Andre Lemburg
CEO eGenix.com Software GmbH
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