linux-ext4 - Re: Weird jbd2 I/O load

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Date:	Tue, 22 Oct 2013 01:11:53 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Andrei Banu <andrei.banu@...host.ro>
CC:	linux-ext4@...r.kernel.org
Subject: Re: Weird jbd2 I/O load

On 10/22/2013 12:55 AM, Zheng Liu wrote:
> On 10/21/2013 10:24 PM, Andrei Banu wrote:
>> Hi Zheng,
>>
>> Thank you for your reply. We can make this test and if it doesn't help
>> we'll re-enable
>> the barrier but first I need to ask a few questions:
>>
>> 1. /dev/md2 is mounted on /. So your command should look like this?
>>
>> $ mount -t ext4 -o remount,barrier=0    /dev/md2    /
> 
> Yes.
> 
>>
>> In /etc/fstab I have other parameters as well:
>> noatime,usrjquota=quota.user,jqfmt=vfsv0
>> Do I also add these like this:
>>
>> $ mount -t ext4 -o
>> remount,barrier=0,noatime,usrjquota=quota.user,jqfmt=vfsv0 /dev/md2     /
> 
> If I remember correctly, this command is OK.  But frankly I don't do the
> test.  I am not sure all these options will pass when you remount a
> partition.
> 
>>
>> 2. Can the command above be run on an active (very active) cPanel server?
> 
> Sorry, I am not familiar with cPanel server.  But if your previous
> testings run on a server under a heavy pressure.  It quite impacts the
> result because disk bandwidth is occupied by this process.

Ah, sorry, I misunderstand your question.  Yes, you can remount a
partition on an active server.  But as I said above, please do not run
your testings on a server under heavy pressure because the result could
be impacted.

Regards,
						- Zheng

> 
>>
>> 3. How do I re-enable the barrier?
> 
> sudo mount -t ext4 -o remount,barrier=1 ${DEV} ${MNT}
> 
>>
>> 4. What is the probability of data loss in case of cold reboot?
> 
> After disabling barrier, the system couldn't ensure that the dirty data
> has been written into the disk medium if your device couldn't support
> FUA command.  So I just want to make sure that your SSD can not handle
> barrier command properly.  *Please do not use 'barrier=0' in your
> product system*.
> 
> 						- Zheng
> 
>>
>> Thanks!
>>
>> On 10/21/2013 4:53 PM, Zheng Liu wrote:
>>> Hi Andrei,
>>>
>>> Could you please disable barrier for ext4 and try your 'dd' test again?
>>>    $ sudo mount -t ext4 -o remount,barrier=0 ${DEV} ${MNT}
>>>
>>> *WARNING: you could lost your data with barrier=0 when you get a power
>>> failure or cold reset.*
>>>
>>> We have met a similar problem that is because some SSDs couldn't handle
>>> barrier command properly.
>>>
>>> Regards,
>>>                                                  - Zheng
>>
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