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Date:	Mon, 30 Jan 2012 15:50:24 -0700
From:	Andreas Dilger <adilger@...ger.ca>
To:	Dave Chinner <david@...morbit.com>
Cc:	Tao Ma <tm@....ma>, Robin Dong <hao.bigrat@...il.com>,
	Ted Ts'o <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [RFC] Add new extent structure in ext4

On 2012-01-29, at 3:07 PM, Dave Chinner wrote:
> On Fri, Jan 27, 2012 at 10:27:02PM +0800, Tao Ma wrote:
>> Hi Dave,
>> On 01/27/2012 08:19 AM, Dave Chinner wrote:
>>> On Wed, Jan 25, 2012 at 04:03:09PM -0700, Andreas Dilger wrote:
>>>> On 2012-01-25, at 3:48 PM, Dave Chinner wrote:
>>>>> On Mon, Jan 23, 2012 at 08:51:53PM +0800, Robin Dong wrote:
>>>>>> Hi Ted, Andreas and the list,
>>>>>> 
>>>>>> After the bigalloc-feature is completed in ext4, we could have much more
>>>>>> big size of block-group (also bigger continuous space), but the extent
>>>>>> structure of files now limit the extent size below 128MB, which is not
>>>>>> optimal.
> 
> .....
> 
>>>>>> The new extent format could support 16TB continuous space and larger volumes.
>>>>>> 
>>>>>> What's your opinion?
>>>>> 
>>>>> Just use XFS.
>>>> 
>>>> Thanks for your troll.
>>>> 
>>>> If you have something actually useful to contribute, please feel free to post.
>>>> Otherwise, this is a list for ext4 development.
>>> 
>>> You can chose to see my comment as a troll, but it has a serious
>>> message. If that is your use case is for large multi-TB files, then
>>> why wouldn't you just use a filesystem that was designed for files
>>> that large from the ground up rather than try to extend a filesystem
>>> that is already struggling with file sizes that it already supports?
>>> Not to mention that very few people even need this functionality,
>>> and those that do right now are using XFS.
>> 
>> Robin is one of my colleague. And to be frank, ext4 works well currently
>> in our product system. And we'd like to see it grows to fit our future
>> need also.
> 
> Sure. But at the expense of the average user? ext4 is supposed to be
> primarily the Linux desktop filesystem,

That is your opinion, as an XFS developer that is trying to keep XFS
relevant for some part of the market.  Yet ext4 does extremely well
at both the desktop and server workloads.

> yet all I see is people trying to make it something for big, bigger
> and biggest. Bigalloc, new extent formats, no-journal mode,
> dioread_nolock, COW snapshots, secure delete, etc. It's a list of
> features that are somewhat incompatible with each other that are
> useful to only a handful of vendors or companies. Most have no
> relevance at all to the uses of the majority of ext4 users.

???  This is quickly degrading into a mud slinging match.  You claim
that "because ext4 is only relevant for desktops, it shouldn't try to
scale or improve performance".  Should I similarly claim that "because
XFS is only relevant to gigantic SMP systems with huge RAID arrays it
shouldn't try to improve small file performance or be CPU efficient"?

Not at all.  The ext4 users and developers choose it because it meets
their needs better than XFS for one reason or another, and we will
continue to improve it for everyone while we are interested to do so.
The ext4 multi-block allocator was originally done for high-throughput
file servers, but it is totally relevant for desktop workloads today.
The same is true for delayed allocation, and other improvements in the
past.  I imagine that bigalloc would be very welcome for media servers
and other large file IO environments.

> This is what I'm getting at - I don't object to adding functionality
> that is generically useful and applies to all filesystem configs,
> but that's not what is happening. ext4 appears to have a development
> mindset of "if we don't support X, then we can do Y" and I don't
> think that serves the ext4 users very well at all.
> 
> BTW, if you think that is a harsh criticism, just reflect on the
> insanity of the recent "we can support 64k block sizes if we just
> disable mmap" discussion. Yes, that's great for Lustre, but it is
> useless for everyone else...

I don't see that at all.  The complexity of blocksize > PAGE_SIZE
is greatly reduced if we don't have to support mmap IO.  Of course
I'd be much happier if the VM supported this properly, but it's been
10 years and it hasn't happened, so waiting longer isn't reasonable.

To be honest, I totally agree that large blocks may not be relevant
for every desktop user.  It may not even be relevant for Lustre, but
that isn't a valid reason not even to _discuss_ feature development
and see where that leads us to an implementation that meets a number
of different needs.

Disabling mmap IO for some configurations doesn't prevent someone from
having a 4kB block LV for the root filesystem, and a separate data LV
for large file IO.  It isn't that mmap for blocksize > PAGE_SIZE is
impossible to implement, but I'd rather see the code handling the
real-world use cases (efficient large file IO, filesystem portability
between IA64, PPC, ARM) than growing extra complexity to handle an
obscure use case (e.g. mmap file IO and binaries executed from a data
storage filesystem).

Once we get the mechanics of large block allocation, we can still look
into the complexity of mmap thereon, since a large block ext4 filesystem
does not actually involve a disk format change since it has been handled
for ages by ext2/3/4 for CPUs that have larger PAGE_SIZE.  Handling mmap
was in Robin's original submission, and I suggested that we exclude it
initially to reduce complexity for the initial implementation.

>> I think it helps both the community and our employer. Having
>> said that, another reason why we don't consider of XFS as our choice is
>> that we don't think we have the ability to maintain 2 file systems in
>> our product system.
> 
> That's your choice as a product vendor, not mine as an ext4 user....

You're suggesting that if I started using XFS on my home filesystems
then I get veto power over your development plans?  Hmm, I don't think
that is going to happen.  Later on, you claim that you aren't even an
ext4 user, so what is the point of your complaint?

The way it works is that anyone is free to develop any features they want
for ext4, they are free to post them to this list (or not) and the ext4
maintainers can evaluate them on functionality and performance in the
manner that they see fit, without any requirement that they be accepted,
keeping in mind that we _do_ take regular user needs into account.

The mere existence of a feature, nay even the discussion of a feature for
ext4, should not be stifled by the suggestion that XFS is the last word
in filesystems (especially since ZFS has already claimed that label :-).

>>> Indeed, on current measures, a 15.95TB file on ext4 takes 330s to
>>> allocate on my test rig, while XFS will do it under *35
>>> milliseconds*. What's the point of increasing the maximum file size
>>> when it when it takes so long to allocate or free the space? If you
>>> can't make the allocation and freeing scale first to the existing
>>> file size limits, there's little point in introducing support for
>>> larger files.
>> 
>> I think your test case here is biased since you used the most successful
>> story from XFS. Yes, bitmap-based file system is a little bit hard to
>> allocate a very large file if the bitmap is scattered all over the disk,
> 
> Which is the case whenever the filesytem has been used for a while.
> I did those tests on a pristine, empty filesystem, so the speed of
> allocation only goes down from there. bitmap based allocation
> degrades much, much faster than extent-tree based allocation,
> especially when you have to search for the free space to allocation
> from....
> 
> Indeed, how do you plan to test such large files robustly when it
> takes so long to allocate the space to them? I mean, I can easily
> test large files on XFS because of how quickly allocation occurs. I
> can easily fragment free space and test large fragmented files
> bcause of how quickly allocation occurs. But if the same test that
> take a minute to run on XFS take 4 orders of magnitude longer on
> ext4, just how good is your test coverage going to be? What about
> when you have different filesystem block sizes, or different mount
> options, or doing it concurrently with an online resize? 
> 
> IOWs, the slowness of the allocation greatly limits the ability to
> test such a feature at the scale it is designed to support.  That's
> my big, overriding concern - with ext4 allocation being so slow, we
> can't really test large files with enough thoroughness *right now*.
> Increasing the file size is only going to make that problem worse
> and that, to me, is a show stopper. If you can't test it properly,
> then the change should not be made.

Hmm, excellent suggestion.  Maybe if we implement faster allocation
for ext4 your objections could be quieted?  Wait, that is what you
are objecting to in the first place (bigalloc, large blocks, etc) or
any changes to ext4 that don't meet your approval.

>> but I don't think ext4 can't fill the gap of this test case in the
>> future. Let us wait and see. :)
> 
> How do you plan to fix it? If there isn't a plan, or it involves a
> major on-disk format change, then aren't we back to square one about
> adding intrusive, complex and destablising features to a filesystem
> that people are relying to be stable?
> 
>>> And as an ext4 user, all I want is from ext4 to be stable like ext3
>>> is stable, not have it continually destabilised by the addition of
>>> incompatible feature after incompatible feature.  Indeed, I can't
>>> use ext4 in the places I'm using ext3 right now because ext4 is not
>>> very resilient in the face of 20 system crashes a day. I generally
>>> find that ext4 filesystems are irretrievable corrupted within a
>>> week.  In comparison, I have ext3 filesystems have lasted more than
>>> 3 years under such workloads without any corruptions occurring.
>> 
>> OK, so next time when you see the corruption, please at least send it to
>> the mail list so that ext4 developers can have the chance of seeing it.
>> Complaint doesn't improve it.
> 
> I won't be reporting corruptions because I stopped using ext4 more
> than 6 months ago on these machines after the last batch of
> unreproducable, unrepairable corruptions that occurred.  I couldn't
> get anything from the corpses (I do know how to analyse a corrupt
> ext4 filesystem), so there really wasn't anything to report....
> 
> Generally speaking, the first sign of problems was a corrupted
> binary or missing or empty file. The filesystem never complained or
> detected corruption at runtime. By that stage, the original cause of
> the corruption was unfindable because the problems may have happened
> many crashes ago and been propagated further. running e2fsck at that
> point generally resulted in a mess with lots of stuff ending in
> lost+found and multiply linked blocks being duplicated all over the
> place. IOWs, an unrecoverable mess.

I haven't heard of similar problems reported here, but even the
existence of such bug reports can be useful alert developers about
the existence of such a problem, and to help narrow down corruption
issues to a specific kernel version.

>>> So the long form of my 3-word comment is effectively: "If you need
>>> multi-TB files, then use the filesystem most appropriate for that
>>> workload instead of trying to make ext4 more complex and unstable
>>> than it already is".
>> 
>> I have read and watched the talk you gave in this year's LCA, your
>> assumption about ext4 may be a little frightening, but it is good for
>> the ext4 community. In your talk "xfs is much slower than ext4 in
>> 2009-2010 for meta-intensive workload", and now it works much faster. So
>> why do you think ext4 can't be improved also like xfs?
> 
> Because all of the XFS changes talked about in that talk did not
> change the on-disk format at all. They are *software-only* changes
> and are completely transparent to users. They are even the default
> behaviours now, so users with 10 year old XFS filesystems will also
> benefit from them. And they can go back to their old kernels if they
> don't like the new kernels, too...

That is only partly true.  XFS had to change the 32-bit vs. 64-bit
inode numbers to get better performance, and that is not backward
compatible on 32-bit systems.  XFS had changed the logging format
to be more efficient in order to not suck at metadata benchmarks.

> We know that the problems ext4 has are much, much deeper and as this
> thread shows require significant on-disk format changes to solve.

That is a very broad statement, and I think it is your extrapolation
from reading a snippet of one thread on this list.

> And they will only benefit those that have new filesystems or make
> their old filesystems incompatible with old kernels. IOWs, the
> changes being proposed don't help solve problems on all the existing
> filesystems transparently.  That's a *major* difference between
> where XFS was 2 years ago and where ext4 is now.

Not true.  The ext4 code can mount and run ancient ext2 filesystems
and shows a significant performance improvement without any on-disk
format changes.  Ask google about their million(?) ext4 filesystems
and how they have improved with only a software update.

Maybe the converse could also be said, that the fact that XFS can
show so much performance improvement without changing the on-disk
format is a testament to how complex and badly written the old code
was?  I think that argument holds as little value as yours, but I
don't jump up and down in xfs@....sgi.com touting the fact that
ext4 is as fast as (or faster than) XFS for most real-world workloads
with only 1/2 of the code.

> Sure, given enough time and resources, any problem is solvable. But
> really, do ext4 users really need a new, incompatible, difficult to
> test on-disk formats to solve problems that most people will never
> hit on their desktop and server systems before they migrate them to
> BTRFS?

Again, you are entitled to your opinion, and are free to spend your
time and efforts where you like.  I wish Chris all the best for Btrfs,
but having looked at that code I'm not in a hurry to move over to
using it for our production workloads, nor even for my home file server.

The joy of open source software is that everyone is free to make their
own choices.  I've made mine, and along with many other developers and
users the choice has been ext4.  Thanks for your input, we'll continue
to discuss and develop whatever we want, regardless of how much you
want everyone to use XFS.

Cheers, Andreas





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