Re: How to flush the disk write cache from userspace

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Ricardo Correia wrote:
On Tuesday 16 January 2007 00:38, you wrote:
As always with these things, the devil is in the details. It requires
the device to support a ->prepare_flush() queue hook, and not all
devices do that. It will work for IDE/SATA/SCSI, though. In some devices
you don't want/need to do a real disk flush, it depends on the write
cache settings, battery backing, etc.

Is there any chance that someone could implement this (I don't have the skills, unfortunately)? Maybe add a new ioctl() to block devices, so that it doesn't break any existing code?

I think we really should have support for doing cache flushes automatically on fsync, etc. User space code should not have to worry about this problem, it's pretty silly that for example MySQL has to advise people to use hdparm -W 0 to disable the write cache on their IDE drives in order to get proper data integrity guarantees - and disabling the cache on IDE without command queueing really slaughters the performance, unnecessarily in this case.

There may be some cases where the controller provides a battery-backed cache and thus we don't want to actually force the controller to flush everything out to the drive on fsync, so we may need to be able to disable this, but these controllers may ignore flushes anyway. I know IBM ServeRAID appears to fail requests for write cache info and so the kernel assumes drive cache: write through and doesn't do any flushes.


I believe it's a very useful (and relatively simple) feature that increases data integrity and reliability for applications that need this functionality.

I think it must be considered that most people have disk write caches enabled and are using IDE, SATA or SCSI disks.

I also think there's no point in disabling disks' write caches, since it slows writes and decreases disks' lifetime, and because there's a better solution.

Yes, ideally doing all writes to the drive with write cache enabled and then flushing them out afterwards would be much more efficient (at least when no command queueing is involved) since the drive can choose what order to complete the writes in.


Personally, I'm not really interested in specific filesystem behaviour, since my application uses block devices directly (it's a filesystem itself). Although I think all filesystems should guarantee data integrity in the face of fsync() or metadata modifications, even if it costs a little performance.

--
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from [email protected]
Home Page: http://www.roberthancock.com/

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