Re: Incredibly poor performance of mdraid-1 with 2 SSD Samsung 840 PRO

[Andrei Banu <andrei.banu@redhost.ro>]

Hi,

I don't have fstab discard option set. I was just enumerating the trim 
kinds I know. I did try discard but it didn't do anything good. And the 
problem dated from before my discard test.

Regards!

On 2013-04-25 00:53, Roberto Spadim wrote:
> TRIM in ext4 = discard
> 2013/4/24 Andrei Banu <andrei.banu@redhost.ro>
> 
>> Hi,
>> 1. How can I at least start trying to find the daemon that might be 
>> doing this?
>> 2. I am not sure what real time TRIM is. I thought there was the 
>> 'discard' option in
>> fstab (which I tried and didn't help) and other command like trims 
>> (fstrim - which
>> errors out when run on / or mdtrim that seems somebody's experiment). 
>> But I
>> am not sure what real time trim might be.
>> I am not really sure where do I go from here. I am a bit lost as it 
>> seems we hit
>> a dead end.
>> Thanks!
>> Andrei Banu
>> On 24/04/2013 7:37 PM, Stan Hoeppner wrote:
>> 
>>> On 4/24/2013 3:26 AM, Andrei Banu wrote:
>>> 
>>>> Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s
>>>>    TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO> 
>>>> COMMAND
>>>>    541 be/3 root        0.00 B/s    0.00 B/s  0.00 % 96.96 % 
>>>> [jbd2/md2-8]
>>> This seems to be your problem.  jbd2 (journal block device) is 
>>> causing
>>> 97% iowait, yet without doing much physical IO.  This is a component 
>>> of
>>> EXT4.  As this will fire intermittently it explains why you see such 
>>> a
>>> wide throughput gap between tests at different points in time.
>>> This isn't a bug or Google would reveal that.  Andrei, you need to
>>> identify which daemon or kernel feature is causing this.  Do you 
>>> happen
>>> to have realtime TRIM enabled?  It is well known to bring IO to a 
>>> crawl.
>>> If not realtime TRIM, I'd guess you turned a knob you should not 
>>> have in
>>> some config file, causing a daemon to frequently issue a few 
>>> gazillion
>>> atomic updates.
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> --
> Roberto Spadim
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