Re: Ext3 behavior on power failure

Re: Ext3 behavior on power failure

[Ric Wheeler]

armangau_philippe@emc.com wrote:
> Hi all,
>
> We are building a new system which is going to use ext3 FS. We would like to know more about the behavior of ext3 in the case of failure.  But before I procede, I would like to share more information about our future system. 
>
> *	Our application always does an fsync on files
> *	When symbolic links (more specifically fast symlink) are created, the host directory is also fsync'ed. 
> *	Our application is also going to front an EMC disk array configured using RAID5 or RAID6.
> *	We will be using multipathing  so that we can assume that no disk errors will be reported. 
>
> In this context , we would like to know the following for recovery after a power outage:
>
> 1.	When will an fsck have to be run (not counting  the scheduled fsck every N-mounts)?
> 2.	In the case of a crash, are the fsync-ed file contents and symbolic links safe no matter what?
>
> Thanks,

This is an interesting twist on some of the discussion that we have had 
at the recent workshop and in other forums on hardening  file system in 
order to prevent the need to fsck.

The twist is that we have a disk that will not lose power without being 
able to write to platter all of the data that has been sent - this is 
the case for most mid-range or higher disk arrays.

If the application can precisely use fsync() on files, directories and 
symlinks, it wants to know that all objects are safe on disk that have 
completed a successful fsync. It also wants to know that the file system 
will not need any recovery beyond replaying transactions after a power 
outage/reboot - simply mount, let the transactions get replayed and you 
should be good to go without the fsck.

The hard part of the question is to understand when and how often we 
will fail to deliver this easy case. Also, does any of the hardening in 
ext4 help here.

Maybe the Stanford explode work/analysis sheds some light on this behavior?

ric

Re: Ext3 behavior on power failure

[Jan Kara]

> armangau_philippe@emc.com wrote:
> >Hi all,
> >
> >We are building a new system which is going to use ext3 FS. We would like 
> >to know more about the behavior of ext3 in the case of failure.  But 
> >before I procede, I would like to share more information about our future 
> >system. 
> >*	Our application always does an fsync on files
> >*	When symbolic links (more specifically fast symlink) are created, 
> >the host directory is also fsync'ed. *	Our application is also 
> >going to front an EMC disk array configured using RAID5 or RAID6.
> >*	We will be using multipathing  so that we can assume that no disk 
> >errors will be reported. 
> >In this context , we would like to know the following for recovery after a 
> >power outage:
> >
> >1.	When will an fsck have to be run (not counting  the scheduled fsck 
> >every N-mounts)?
> >2.	In the case of a crash, are the fsync-ed file contents and symbolic 
> >links safe no matter what?
> >
> >Thanks,
> 
> This is an interesting twist on some of the discussion that we have had 
> at the recent workshop and in other forums on hardening  file system in 
> order to prevent the need to fsck.
> 
> The twist is that we have a disk that will not lose power without being 
> able to write to platter all of the data that has been sent - this is 
> the case for most mid-range or higher disk arrays.
> 
> If the application can precisely use fsync() on files, directories and 
> symlinks, it wants to know that all objects are safe on disk that have 
> completed a successful fsync. It also wants to know that the file system 
> will not need any recovery beyond replaying transactions after a power 
> outage/reboot - simply mount, let the transactions get replayed and you 
> should be good to go without the fsck.
> 
> The hard part of the question is to understand when and how often we 
> will fail to deliver this easy case. Also, does any of the hardening in 
> ext4 help here.
  I'm probably misunderstanding something because the answer seems to be
too obvious to me :) But anyway I'll write it so that you can correct
me:
  Due to journalling guarantees you should get consistent FS whenever
you replay the log (unless there are some software bugs or hardware
problems which is why fsck is run once per several mounts anyway).
  If you fsync() your data, you are guaranteed that also your data are
safely on disk when fsync returns. So what is the question here?

								Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs

Re: Ext3 behavior on power failure

[John Anthony Kazos Jr.]

>  If you fsync() your data, you are guaranteed that also your data are
> safely on disk when fsync returns. So what is the question here?

Pardon a newbie's intrusion, but I do know this isn't true. There is a 
window of possible loss because of the multitude of layers of caching, 
especially within the drive itself. Unless there is a super_duper_fsync() 
that is able to actually poll the hardware and get a confirmation that the 
internal buffers are purged?

Re: Ext3 behavior on power failure

[Jan Kara]

> > If you fsync() your data, you are guaranteed that also your data are
> >safely on disk when fsync returns. So what is the question here?
> Pardon a newbie's intrusion, but I do know this isn't true. There is a 
> window of possible loss because of the multitude of layers of caching, 
> especially within the drive itself. Unless there is a super_duper_fsync() 
> that is able to actually poll the hardware and get a confirmation that the 
> internal buffers are purged?
  OK :), to correct myself: After fsync() returns, all the data is acked from
the disk (or at least it should be like that unless there's a bug
somewhere). So if there are some caches in the hardware which the hardware
is not able to flush on power failure, that's a bad luck... That's why
you should turn off write caching on cheaper disks if you really care
about data integrity.

									Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs

RE: Ext3 behavior on power failure

[armangau_philippe]

In my case the disk cache is not a  problem - We use an emc disk array
the write cache is protected - 
Once the data has made over the disk array we can assume it is safe - 
Thx 
Philippe  

-----Original Message-----
From: John Anthony Kazos Jr. [mailto:jakj@j-a-k-j.com] 
Sent: Wednesday, March 28, 2007 9:17 AM
To: Jan Kara
Cc: wheeler, richard; armangau, philippe; ext3-users@redhat.com;
linux-ext4@vger.kernel.org; csar@stanford.edu
Subject: Re: Ext3 behavior on power failure

>  If you fsync() your data, you are guaranteed that also your data are
> safely on disk when fsync returns. So what is the question here?

Pardon a newbie's intrusion, but I do know this isn't true. There is a 
window of possible loss because of the multitude of layers of caching, 
especially within the drive itself. Unless there is a
super_duper_fsync() 
that is able to actually poll the hardware and get a confirmation that
the 
internal buffers are purged?

Re: Ext3 behavior on power failure

[Jan Kara]

On Wed 28-03-07 10:17:33, armangau_philippe@emc.com wrote:
> In my case the disk cache is not a  problem - We use an emc disk array
> the write cache is protected - 
> Once the data has made over the disk array we can assume it is safe - 
  Then if you are able to reproduce the situation that not all data
is written after fsync(); poweroff; that is a bug worth reporting..

								Honza
> 
> -----Original Message-----
> From: John Anthony Kazos Jr. [mailto:jakj@j-a-k-j.com] 
> Sent: Wednesday, March 28, 2007 9:17 AM
> To: Jan Kara
> Cc: wheeler, richard; armangau, philippe; ext3-users@redhat.com;
> linux-ext4@vger.kernel.org; csar@stanford.edu
> Subject: Re: Ext3 behavior on power failure
> 
> >  If you fsync() your data, you are guaranteed that also your data are
> > safely on disk when fsync returns. So what is the question here?
> 
> Pardon a newbie's intrusion, but I do know this isn't true. There is a 
> window of possible loss because of the multitude of layers of caching, 
> especially within the drive itself. Unless there is a
> super_duper_fsync() 
> that is able to actually poll the hardware and get a confirmation that
> the 
> internal buffers are purged?
> 
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs

Re: Ext3 behavior on power failure

[Bruno Wolff III]

On Wed, Mar 28, 2007 at 09:17:27 -0400,
  "John Anthony Kazos Jr." <jakj@j-a-k-j.com> wrote:
> > If you fsync() your data, you are guaranteed that also your data are
> >safely on disk when fsync returns. So what is the question here?
> 
> Pardon a newbie's intrusion, but I do know this isn't true. There is a 
> window of possible loss because of the multitude of layers of caching, 
> especially within the drive itself. Unless there is a super_duper_fsync() 
> that is able to actually poll the hardware and get a confirmation that the 
> internal buffers are purged?

That is why you need to disable write caching of the drives or use cache
flushes via write barriers (if the stack of block devices all support them)
if the hardware cache isn't battery backed or the device doesn't support
returning the status of particular commands.

Of course nothing is perfectly safe.

Re: Ext3 behavior on power failure

[Ric Wheeler]

Jan Kara wrote:
>> armangau_philippe@emc.com wrote:
>>> Hi all,
>>>
>>> We are building a new system which is going to use ext3 FS. We would like 
>>> to know more about the behavior of ext3 in the case of failure.  But 
>>> before I procede, I would like to share more information about our future 
>>> system. 
>>> *	Our application always does an fsync on files
>>> *	When symbolic links (more specifically fast symlink) are created, 
>>> the host directory is also fsync'ed. *	Our application is also 
>>> going to front an EMC disk array configured using RAID5 or RAID6.
>>> *	We will be using multipathing  so that we can assume that no disk 
>>> errors will be reported. 
>>> In this context , we would like to know the following for recovery after a 
>>> power outage:
>>>
>>> 1.	When will an fsck have to be run (not counting  the scheduled fsck 
>>> every N-mounts)?
>>> 2.	In the case of a crash, are the fsync-ed file contents and symbolic 
>>> links safe no matter what?
>>>
>>> Thanks,
>> This is an interesting twist on some of the discussion that we have had 
>> at the recent workshop and in other forums on hardening  file system in 
>> order to prevent the need to fsck.
>>
>> The twist is that we have a disk that will not lose power without being 
>> able to write to platter all of the data that has been sent - this is 
>> the case for most mid-range or higher disk arrays.
>>
>> If the application can precisely use fsync() on files, directories and 
>> symlinks, it wants to know that all objects are safe on disk that have 
>> completed a successful fsync. It also wants to know that the file system 
>> will not need any recovery beyond replaying transactions after a power 
>> outage/reboot - simply mount, let the transactions get replayed and you 
>> should be good to go without the fsck.
>>
>> The hard part of the question is to understand when and how often we 
>> will fail to deliver this easy case. Also, does any of the hardening in 
>> ext4 help here.
>   I'm probably misunderstanding something because the answer seems to be
> too obvious to me :) But anyway I'll write it so that you can correct
> me:
>   Due to journalling guarantees you should get consistent FS whenever
> you replay the log (unless there are some software bugs or hardware
> problems which is why fsck is run once per several mounts anyway).
>   If you fsync() your data, you are guaranteed that also your data are
> safely on disk when fsync returns. So what is the question here?
> 
> 								Honza

I think that the real question here is in practice, how often does this really 
hold to be true? When it fails, how long does it take to recover the file system?

There are a lot of odd errors that can happen when you monitor a large enough 
number of file systems. In my experience, I would guess that disk errors are 
clearly the leading cause of issues, followed by software bugs (file system, 
firmware, etc) and then a group of errors caused by various occasional things 
(bad DRAM in the server/HBA/disk, bad cables/etc). Note that using a high end 
array does not eliminate errors, it just reduces the rate (hopefully by a large 
amount).

What is really hard to predict is the rate of the failures that require fsck 
with our current file system (say for a specific hardware setup) and how changes 
like the checksumming in ext4 can help us ride through these errors without 
needing a full fsck.

This rate has a direct impact on how much pain an fsck will inflict and how 
important redundancy is to avoid having the file system be a single point of 
failure.

ric

Re: Ext3 behavior on power failure

[Jan Kara]

On Wed 28-03-07 19:00:54, Ric Wheeler wrote:
> Jan Kara wrote:
> >>armangau_philippe@emc.com wrote:
> >>>Hi all,
> >>>
> >>>We are building a new system which is going to use ext3 FS. We would 
> >>>like to know more about the behavior of ext3 in the case of failure.  
> >>>But before I procede, I would like to share more information about our 
> >>>future system. 
> >>>*	Our application always does an fsync on files
> >>>*	When symbolic links (more specifically fast symlink) are created, 
> >>>the host directory is also fsync'ed. *	Our application is also 
> >>>going to front an EMC disk array configured using RAID5 or RAID6.
> >>>*	We will be using multipathing  so that we can assume that no disk 
> >>>errors will be reported. 
> >>>In this context , we would like to know the following for recovery after 
> >>>a power outage:
> >>>
> >>>1.	When will an fsck have to be run (not counting  the scheduled fsck 
> >>>every N-mounts)?
> >>>2.	In the case of a crash, are the fsync-ed file contents and symbolic 
> >>>links safe no matter what?
> >>>
> >>>Thanks,
> >>This is an interesting twist on some of the discussion that we have had 
> >>at the recent workshop and in other forums on hardening  file system in 
> >>order to prevent the need to fsck.
> >>
> >>The twist is that we have a disk that will not lose power without being 
> >>able to write to platter all of the data that has been sent - this is 
> >>the case for most mid-range or higher disk arrays.
> >>
> >>If the application can precisely use fsync() on files, directories and 
> >>symlinks, it wants to know that all objects are safe on disk that have 
> >>completed a successful fsync. It also wants to know that the file system 
> >>will not need any recovery beyond replaying transactions after a power 
> >>outage/reboot - simply mount, let the transactions get replayed and you 
> >>should be good to go without the fsck.
> >>
> >>The hard part of the question is to understand when and how often we 
> >>will fail to deliver this easy case. Also, does any of the hardening in 
> >>ext4 help here.
> >  I'm probably misunderstanding something because the answer seems to be
> >too obvious to me :) But anyway I'll write it so that you can correct
> >me:
> >  Due to journalling guarantees you should get consistent FS whenever
> >you replay the log (unless there are some software bugs or hardware
> >problems which is why fsck is run once per several mounts anyway).
> >  If you fsync() your data, you are guaranteed that also your data are
> >safely on disk when fsync returns. So what is the question here?
> >
> >								Honza
> 
> I think that the real question here is in practice, how often does this 
> really hold to be true? When it fails, how long does it take to recover the 
> file system?
  I see, thanks for explanation :).

> There are a lot of odd errors that can happen when you monitor a large 
> enough number of file systems. In my experience, I would guess that disk 
> errors are clearly the leading cause of issues, followed by software bugs 
> (file system, firmware, etc) and then a group of errors caused by various 
> occasional things (bad DRAM in the server/HBA/disk, bad cables/etc). Note 
> that using a high end array does not eliminate errors, it just reduces the 
> rate (hopefully by a large amount).
> 
> What is really hard to predict is the rate of the failures that require 
> fsck with our current file system (say for a specific hardware setup) and 
> how changes like the checksumming in ext4 can help us ride through these 
> errors without needing a full fsck.
  OK. All the features I've seen so far were more aiming to detecting that
such an unexpected problem happened rather than trying to fix it or make
fixing it faster. So currently it seems to me that any such unexpected
failure requires fsck...

								Honza
-- 
Jan Kara <jack@suse.cz>
SuSE CR Labs