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Old 11-26-2010, 12:31 PM
Ben Hutchings
 
Default The fsync issue

Just got this from Christoph Helwig:

13:23 < hch> bwh: if you guys are interested in helping dpkg review and ack the
per-fs sync ioctl path that sage weil sent out a couple of weeks
ago
13:24 < hch> bwh: and report the ext4 fsync issues to the list, I know ext4
fsync isn't stellar, but the numbers sounds so bad that there must
be a bug somewhere

The patch referred to is in
<http://thread.gmane.org/gmane.linux.file-systems/44628>.

Ben.

--
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.
 
Old 11-27-2010, 05:59 AM
Guillem Jover
 
Default The fsync issue

Hi Ben!

On Fri, 2010-11-26 at 13:31:20 +0000, Ben Hutchings wrote:
> Just got this from Christoph Helwig:
>
> 13:23 < hch> bwh: if you guys are interested in helping dpkg review and ack the
> per-fs sync ioctl path that sage weil sent out a couple of weeks
> ago
> 13:24 < hch> bwh: and report the ext4 fsync issues to the list, I know ext4
> fsync isn't stellar, but the numbers sounds so bad that there must
> be a bug somewhere

> The patch referred to is in
> <http://thread.gmane.org/gmane.linux.file-systems/44628>.

Unfortunately that patch does not seem much appealing, it's Linux only,
not even in mainline, and it would need for dpkg to track on which file
system each file is located and issue such ioctl once per file system.

I'd rather not complicate the dpkg source code even more for something
that seems to me to be a bug or missfeature in the file system. More so
when there's a clear fix (nodelalloc) that solves both the performance
and data safety issues in general.

thanks,
guillem


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Old 11-27-2010, 06:41 AM
Jonathan Nieder
 
Default The fsync issue

Hi Guillem,

Guillem Jover wrote:

> Unfortunately that patch does not seem much appealing, it's Linux only,
> not even in mainline, and it would need for dpkg to track on which file
> system each file is located and issue such ioctl once per file system.
>
> I'd rather not complicate the dpkg source code even more for something
> that seems to me to be a bug or missfeature in the file system. More so
> when there's a clear fix (nodelalloc) that solves both the performance
> and data safety issues in general.

I don't really understand this point of view: isn't the fsync storm
going to cause seeky I/O on just about all file systems?

So the POSIX primitives are not rich enough to express what we want to
happen. Delayed allocation is pretty much essential for the use case
ubifs targets, so it doesn't make much sense to me to pretend it
doesn't exist.

I'll look into a (Linux-specific, obviously) patch to add a function
that takes an array of paths and performs the relevant syncs of
filesystems where that ioctl exists tomorrow. I would rather see a
system call that just takes an array of paths, since I imagine
filesystems like btrfs could do something good with that, but since
there are no VFS primitives for it I can see why that wasn't proposed.

Regards,
Jonathan


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