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-   -   To those moving to Jaunty: Do not use the new ext4 (http://www.linux-archive.org/kubuntu-user/261805-those-moving-jaunty-do-not-use-new-ext4.html)

Jonas Norlander 03-12-2009 07:45 AM

To those moving to Jaunty: Do not use the new ext4
 
2009/3/12 Dotan Cohen <dotancohen@gmail.com>:
> I'm not blaming the ext4 developers nor am I blaming KDE, but there
> exists a dangerous condition for using KDE under ext4:
> http://linux.slashdot.org/article.pl?sid=09/03/11/2031231
>
> The /. article links to Launchpad bugs. I do not link to them directly
> because the /. comments are actually very important for this issue,
> and will be more informative than the flames and
> responsibility-denying that I am sure will ensue here.
>
> In short, don't use ext4 on Kubuntu Jaunty. In fact, I have argued
> before that end users should avoid ext4 or any new technology for some
> time after debut.
>
> --
> Dotan Cohen

I haven't read all comments and I'm no expert on file systems but as
far as i know ext4 is a journaling file system and by that it should
guarantee that only complete writes of a file should be registered and
if thats not the case the old file should be intact and not truncated.
Apparently thats not the case with ext4 so what have I missed?

This is a showstopper for ext4 in Jaunty as I see it and they should
remove the option for using ext4 at installation.

/ Jonas

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Dotan Cohen 03-12-2009 07:55 AM

To those moving to Jaunty: Do not use the new ext4
 
> I haven't read all comments and I'm no expert on file systems but as
> far as i know ext4 is a journaling file system and by that it should
> guarantee that only complete writes of a file should be registered and
> if thats not the case the old file should be intact and not truncated.
> Apparently thats not the case with ext4 so what have I missed?
>

No, writes are cached. Allegedly, KDE is depending on the cache and
not syncing. This is bad when the write delay is measured in minutes
(ext4) as opposed to seconds (ext3).

> This is a showstopper for ext4 in Jaunty as I see it and they should
> remove the option for using ext4 at installation.
>

Don't tell me, mention that on the bug!

--
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א-ב-ג-ד-ה-ו-ז-ח-ט-י-ך-כ-ל-ם-מ-ן-*-ס-ע-ף-פ-ץ-צ-ק-ר-ש-ת
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Jussi Kekkonen 03-12-2009 08:16 AM

To those moving to Jaunty: Do not use the new ext4
 
FYI it's not only or specifically KDE issue, so no need to crucify
that way, dear thread members

2009/3/12, Dotan Cohen <dotancohen@gmail.com>:
>> I haven't read all comments and I'm no expert on file systems but as
>> far as i know ext4 is a journaling file system and by that it should
>> guarantee that only complete writes of a file should be registered and
>> if thats not the case the old file should be intact and not truncated.
>> Apparently thats not the case with ext4 so what have I missed?
>>
>
> No, writes are cached. Allegedly, KDE is depending on the cache and
> not syncing. This is bad when the write delay is measured in minutes
> (ext4) as opposed to seconds (ext3).
>
>> This is a showstopper for ext4 in Jaunty as I see it and they should
>> remove the option for using ext4 at installation.
>>
>
> Don't tell me, mention that on the bug!
>
> --
> Dotan Cohen
>
> http://what-is-what.com
> http://gibberish.co.il
>
> א-ב-ג-ד-ה-ו-ז-ח-ט-י-ך-כ-ל-ם-מ-ן-*-ס-ע-ף-פ-ץ-צ-ק-ר-ש-ת
> ا-ب-ت-ث-ج-*-خ-د-ذ-ر-ز-س-ش-ص-ض-ط-ظ-ع-غ-ف-ق-ك-ل-م-ن-ه*-و-ي
> А-Б-В-Г-Д-Е-Ё-Ж-З-И-Й-К-Л-М-Н-О-П-*-С-Т-У-Ф-Х-Ц-Ч-Ш-Щ-Ъ-Ы-Ь-*-Ю-Я
> а-б-в-г-д-е-ё-ж-з-и-й-к-л-м-н-о-п-р-с-т-у-ф-х-ц-ч-ш-щ-ъ-ы-ь-э-ю-я
> ä-ö-ü-ß-Ä-Ö-Ü
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Jussi Kekkonen

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Dotan Cohen 03-12-2009 08:20 AM

To those moving to Jaunty: Do not use the new ext4
 
2009/3/12 Jussi Kekkonen <tmt@ubuntu.com>:
> FYI it's not only or specifically KDE issue, so no need to crucify
> that way, dear thread members
>

Right, I should have made that clear. KDE is a prominent application
that is affected by the issue, but it is _not_ KDE exclusive. Any app
that assumes a speedy write without explicitly calling sync() will be
affected.

--
Dotan Cohen

http://what-is-what.com
http://gibberish.co.il

א-ב-ג-ד-ה-ו-ז-ח-ט-י-ך-כ-ל-ם-מ-ן-*-ס-ע-ף-פ-ץ-צ-ק-ר-ש-ת
ا-ب-ت-ث-ج-*-خ-د-ذ-ر-ز-س-ش-ص-ض-ط-ظ-ع-غ-ف-ق-ك-ل-م-ن-ه*-و-ي
А-Б-В-Г-Д-Е-Ё-Ж-З-И-Й-К-Л-М-Н-О-П-*-С-Т-У-Ф-Х-Ц-Ч-Ш-Щ-Ъ-Ы-Ь-*-Ю-Я
а-б-в-г-д-е-ё-ж-з-и-й-к-л-м-н-о-п-р-с-т-у-ф-х-ц-ч-ш-щ-ъ-ы-ь-э-ю-я
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Jonas Norlander 03-12-2009 08:29 AM

To those moving to Jaunty: Do not use the new ext4
 
2009/3/12 Dotan Cohen <dotancohen@gmail.com>:
>> I haven't read all comments and I'm no expert on file systems but as
>> far as i know ext4 is a journaling file system and by that it should
>> guarantee that only complete writes of a file should be registered and
>> if thats not the case the old file should be intact and not truncated.
>> Apparently thats not the case with ext4 so what have I missed?
>>
>
> No, writes are cached. Allegedly, KDE is depending on the cache and
> not syncing. This is bad when the write delay is measured in minutes
> (ext4) as opposed to seconds (ext3).
>

So we would see the same behavior with ext3 if the time between syncs
was the same?

I later read Theodore Ts'o (one of the developer) comment 45 an
explanation why and how:
https://bugs.edge.launchpad.net/ubuntu/+source/linux/+bug/317781/comments/45

But I still cant understand, no matter if you depending on the cache
or the libraries you using don't use flush() the right way isn't a
journaling file system supposed to only register complete transactions
and by that guarantee (of course you cant guarantee that, but it
should be minimal) that the files would not be destroyed/truncated?

/ Jonas

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Dotan Cohen 03-12-2009 08:57 AM

To those moving to Jaunty: Do not use the new ext4
 
> So we would see the same behavior with ext3 if the time between syncs
> was the same?
>

Yes, in fact, there are other filesystems in which this is the case.

> I later read Theodore Ts'o (one of the developer) comment 45 an
> explanation why and how:
> https://bugs.edge.launchpad.net/ubuntu/+source/linux/+bug/317781/comments/45
>
> But I still cant understand, no matter if you depending on the cache
> or the libraries you using don't use flush() the right way isn't a
> journaling file system supposed to only register complete transactions
> and by that guarantee (of course you cant guarantee that, but it
> should be minimal) that the files would not be destroyed/truncated?
>

I am do not really understand all the gotchas of filesystem-level
access, but it seems to me that you would see quite a lot of locked up
processes if a "complete transaction" entailed waiting until the
actual write. The process just needs to know that the filesystem got
the data and will handle it from there. Assuming that the filesystem
will be responsible with that data seems reasonable for most cases.
For critical cases, the process need not assume and can force a write
to disk. Which KDE does not.

--
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http://what-is-what.com
http://gibberish.co.il

א-ב-ג-ד-ה-ו-ז-ח-ט-י-ך-כ-ל-ם-מ-ן-*-ס-ע-ף-פ-ץ-צ-ק-ר-ש-ת
ا-ب-ت-ث-ج-*-خ-د-ذ-ر-ز-س-ش-ص-ض-ط-ظ-ع-غ-ف-ق-ك-ل-م-ن-ه*-و-ي
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а-б-в-г-д-е-ё-ж-з-и-й-к-л-м-н-о-п-р-с-т-у-ф-х-ц-ч-ш-щ-ъ-ы-ь-э-ю-я
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Matthew Flaschen 03-12-2009 02:01 PM

To those moving to Jaunty: Do not use the new ext4
 
Dotan Cohen wrote:
> I'm not blaming the ext4 developers nor am I blaming KDE, but there
> exists a dangerous condition for using KDE under ext4:

It has absolutely nothing to do with KDE, except they are one example of
a app that expects more than POSIX has to offer.

> The /. article links to Launchpad bugs. I do not link to them
> directly because the /. comments are actually very important for this
> issue,

No, they're actually much less important that either the code (
http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=3bf3342f394d72ed2ec7e77b5b 39e1b50fad8284,
http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=6645f8c3bc3cdaa7de4aaa3d34 d40c2e8e5f09ae,
http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=dbc85aa9f11d8c13c15527d43a 3def8d7beffdc8
) or the bug
report
(https://bugs.edge.launchpad.net/ubuntu/+source/linux/+bug/317781). If
you want a clear and concise comment, see
https://bugs.edge.launchpad.net/ubuntu/+source/linux/+bug/317781/comments/45
. Among other things, it notes that ext4's behavior in no way violates
the spec. /However,/ it does conflict with application expectations,
and so we will be adjusted.

> and will be more informative than the flames and
> responsibility-denying that I am sure will ensue here.

It's quite clear the coders are taking responsibility if you actually
bother to read the bug report.

> In short, don't use ext4 on Kubuntu Jaunty. In fact, I have argued
> before that end users should avoid ext4 or any new technology for some
> time after debut.

If this bug is an issue, it's an issue on all DE's. And surely those
taking your advice will not be using development releases in the first
place.

Matt Flaschen

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Matthew Flaschen 03-12-2009 02:02 PM

To those moving to Jaunty: Do not use the new ext4
 
Jonas Norlander wrote:
> I haven't read all comments and I'm no expert on file systems but as
> far as i know ext4 is a journaling file system and by that it should
> guarantee that only complete writes of a file should be registered and
> if thats not the case the old file should be intact and not truncated.

The truncate is an operation requested by the app. It is not a mistake.
The only part that causes problems is the subsequent write (after
truncate) is not synced until too late. ext3 was exactly the same in
this respect, except that it synced much more often.

Matt Flaschen

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Matthew Flaschen 03-12-2009 02:06 PM

To those moving to Jaunty: Do not use the new ext4
 
Jonas Norlander wrote:
> But I still cant understand, no matter if you depending on the cache
> or the libraries you using don't use flush() the right way isn't a
> journaling file system supposed to only register complete transactions
> and by that guarantee (of course you cant guarantee that, but it
> should be minimal) that the files would not be destroyed/truncated?

A journaling system is supposed to ensure that at time n the disk
exactly reflects all disk operations requested prior to some time n-x.
That is true in this case, because the truncate was a /requested/ disk
operation that occurred prior to the write.

Matt Flaschen

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Dotan Cohen 03-12-2009 03:57 PM

To those moving to Jaunty: Do not use the new ext4
 
2009/3/12 Matthew Flaschen <matthew.flaschen@gatech.edu>:
> Dotan Cohen wrote:
>> I'm not blaming the ext4 developers nor am I blaming KDE, but there
>> exists a dangerous condition for using KDE under ext4:
>
> It has absolutely nothing to do with KDE, except they are one example of
> a app that expects more than POSIX has to offer.
>

It has everything to do with KDE because of the way KDE syncs it's data to disk.

There already was some AC on either this list or the Ubuntu list
asking about ext4 and he could not understand that new does not mean
better. I think that the Windows school of though is to use the newest
everything, including beta service packs when they can be torrented.

>> The /. article links to Launchpad bugs. I do not link to them
>> directly because the /. comments are actually very important for this
>> issue,
>
> No, they're actually much less important that either the code (
> http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=3bf3342f394d72ed2ec7e77b5b 39e1b50fad8284,
> http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=6645f8c3bc3cdaa7de4aaa3d34 d40c2e8e5f09ae,
> http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=dbc85aa9f11d8c13c15527d43a 3def8d7beffdc8
> ) or the bug
> report
> (https://bugs.edge.launchpad.net/ubuntu/+source/linux/+bug/317781). *If
> you want a clear and concise comment, see
> https://bugs.edge.launchpad.net/ubuntu/+source/linux/+bug/317781/comments/45
> . Among other things, it notes that ext4's behavior in no way violates
> the spec. */However,/ it does conflict with application expectations,
> and so we will be adjusted.
>

In one month? You trust _your_ data to it.

>> and will be more informative than the flames and
>> responsibility-denying that I am sure will ensue here.
>
> It's quite clear the coders are taking responsibility if you actually
> bother to read the bug report.
>

I meant that uninformed ACs on this list will start throwing blame.
Maybe I underestimated the maturity of this list. Sorry.

>> In short, don't use ext4 on Kubuntu Jaunty. In fact, I have argued
>> before that end users should avoid ext4 or any new technology for some
>> time after debut.
>
> If this bug is an issue, it's an issue on all DE's. *And surely those
> taking your advice will not be using development releases in the first
> place.
>

No, it does ont affect Gnome which syncs it's data in the more robust
method. I do not know about XFCE.

--
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א-ב-ג-ד-ה-ו-ז-ח-ט-י-ך-כ-ל-ם-מ-ן-*-ס-ע-ף-פ-ץ-צ-ק-ר-ש-ת
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