really strange ext4 behavior

Dennis Jacobfeuerborn dennisml at conversis.de
Sun Feb 13 18:29:12 UTC 2011


On 02/12/2011 11:52 PM, Ric Wheeler wrote:
> On 02/12/2011 05:31 PM, Michał Piotrowski wrote:
>> Hi,
>>
>> W dniu 12 lutego 2011 23:19 użytkownik Ric Wheeler
>> <rwheeler at redhat.com>   napisał:
>>> On 02/12/2011 05:12 PM, Michał Piotrowski wrote:
>>>> Hi,
>>>>
>>>> I added a disc to my box. I wanted to use ext4. I run fs_mark to test
>>>> speed, to my surprise I heard a really strange noises.
>>>>
>>>> It's very strange because the drive is new
>>>>     9 Power_On_Hours          0x0032   100   100   000    Old_age
>>>> Always       -       12
>>>>
>>>>
>>>> #  fs_mark  -d  test/
>>>> [..]
>>>> FSUse%        Count         Size    Files/sec     App Overhead
>>>>        0         1000        51200         22.8            54347
>>>>
>>>> I decided to create an ext3 file system on this drive and everything works
>>>> fine.
>>>>
>>>> #  fs_mark  -d  test/
>>>> [..]
>>>> FSUse%        Count         Size    Files/sec     App Overhead
>>>>        0         1000        51200        103.7            57229
>>>>
>>>> When I mount this ext3 fs as ext4 and run fs_mark I hear strange sounds
>>>> again.
>>>>
>>>> I use F14 and self compiled kernel from rawhide 2.6.37-1.fc14.x86_64 +
>>>> e2fsprogs-1.41.14-2.fc14.x86_64.
>>>>
>>>> I mount ecryptfs on top of this file system.
>>>>
>>>> Does anyone know what might be causing this strange ext4 behavior?
>>>>
>>> Hi Michael,
>>>
>>> fs_mark run a fsync heavy test. What you might be hearing is the impact of
>>> the fsync's. ext4 defaults to using "write barriers" enabled, ext3 does not.
>>> Without write barriers, those fsync push data from the box to the write
>>> cache on the drive only. With barriers, the disk will flush that cache to
>>> the platter, so the platter moves and you probably hear the head, etc.
>>>
>>> You can test if this is the cause by mouting ext4 with "nobarrier" to see if
>>> the noise goes away.
>> I mounted fs with nobarrier and now it works just like ext3. Thanks! This solves
>> the riddle :)
>>
>
> Good to hear that it worked!
>
> Note that the barrier code makes your data safer, so you should run with it on
> by default (unless you really don't care about the file system).

If ext3 was running fine without barriers for all these years why is this 
such a problem with ext4? Does ext4 do something differently that barriers 
are now required?

Regards,
   Dennis


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