linux-ext4 - Re: Porting Zfs features to ext2/3

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Date:	Tue, 29 Jul 2008 12:46:29 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Eric Anopolsky <erpo41@...il.com>
CC:	Theodore Tso <tytso@....edu>, postrishi <postrishi@...il.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Porting Zfs features to ext2/3

Eric Anopolsky wrote:
> Please let me know if I'm getting off topic for the ext4-devel list. My
> point is not to advocate ZFS over ext3/4 since ZFS still has its share
> of issues. No resizing raidz vdevs, for example, and performance in
> certain areas. My only point is to make it clear that ZFS on Linux is
> available (and not necessarily a bad choice) to people reading the
> ext4-devel mailing list looking for ZFS-like features like the original
> poster.
>
> On Mon, 2008-07-28 at 08:40 -0400, Theodore Tso wrote:
>   
>> On Sun, Jul 27, 2008 at 10:15:59PM -0600, Eric Anopolsky wrote:
>>     
>>> It's true that ZFS on FUSE performance isn't all it could be right now.
>>> However, ZFS on FUSE is currently not taking advantage of mechanisms
>>> FUSE provides to improve performance. For an example of what can be
>>> achieved, check out http://www.ntfs-3g.org/performance.html .
>>>       
>> Yes... and take a look at the metadata operations numbers.  FUSE can
>> do things to accellerate bulk read/write, but metadata-intensive
>> operations will (I suspect) always be slow.  
>>     
>
> It doesn't seem too much worse than the other non-ext3 filesystems in
> the comparison. I'm sure everyone would prefer a non-FUSE implementation
> and the licensing issues aren't going to go away, but this post on Jeff
> Bonwick's blog gives some hope:
> http://blogs.sun.com/bonwick/entry/casablanca . Even so, not everyone
> needs a whole lot of speed in the metadata operations area. 
>
>   
>> I also question whether
>> the FUSE implementation will have the safety that has always been the
>> Raison d'être of ZFS.  Have you or the ZFS/FUSE developers done tests
>> where you are writing to the filesystem, and then someone pulls the
>> plug on the fileserver while ZFS is writing?  Does the filesystem
>> recovery cleanly from such a scenario?
>>     
>
> I haven't personally tried pulling the plug, but I've tried holding down
> the power button on my laptop until it powers off. Everything works fine
> and scrubs (the closest ZFS gets to fsck) don't report any checksum
> errors. The filesystem driver updates the on-disk filesystem atomically
> every five seconds (less time in special circumstances) so there's never
> any point at which the filesystem would need recovery. The next time the
> filesystem is mounted the system sees the state the filesystem was in up
> to five seconds before the power went out. The FUSEness of the
> filesystem driver doesn't seem to affect this.
>
> Cheers,
> Eric
>   
Does that mean you always lose the last 5 seconds of data before the 
power outage?

We had an earlier thread where Chris had a good test for making a case 
for the write barrier code being enabled by default. It would be neat to 
try that on ZFS ;-) The expected behaviour should be that any fsync()'ed 
files should be there (regardless of the 5 seconds) and other 
non-fsync'ed files might or might not be there, but that all file system 
integrity is complete.

It would also be very interesting to try and do a drive hot pull.

Thanks!

Ric


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