QMail and SoftUpdates

Xin LI delphij at frontfree.net
Mon May 17 19:14:45 PDT 2004

On Mon, May 17, 2004 at 01:25:04PM -0700, Julian Elischer wrote:
> Softupdates is NOT unsafe for this task IF the app does an fsync()
> before acknowldging the incoming mail. Fsync under Soft updates will not
> return until all data and metadata associated with the file
> has been written to disk. This will make the write as slow as a sync 
> munted filesystem, but all other writes on the disk will proceeed at 
> soft-updates speed.

Oh... I missed this point, sorry. Your statement is more accurate, thanks
for pointing this out :-)

I think it should be noted that on ATA drives, the hardware writing
cache (without an acknowledge back to the operating system when the data
is actually written to the disk, like on a tagged queue enabled SCSI
disk), is enabled by default. This will greatly disturb SoftUpdates'
work and may lead to data loss even when the application is correctly
doing fsync(), because with ATA hardware writing cache, the disk will
"cheat" the opearting system - Before the data actually goes to disk,
it tells that it is, and data is lost if crash occours here.

Xin LI <delphij frontfree net>	http://www.delphij.net/
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