linux-kernel - Re: Debugging system freezes on filesystem writes

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Date:	Thu, 12 Sep 2013 15:57:32 +0300
From:	Marcus Sundman <marcus@...ox.fi>
To:	Jan Kara <jack@...e.cz>
CC:	Theodore Ts'o <tytso@....edu>, Dave Chinner <david@...morbit.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Debugging system freezes on filesystem writes

On 27.02.2013 01:17, Jan Kara wrote:
> On Tue 26-02-13 20:41:36, Marcus Sundman wrote:
>> On 24.02.2013 03:20, Theodore Ts'o wrote:
>>> On Sun, Feb 24, 2013 at 11:12:22AM +1100, Dave Chinner wrote:
>>>>>> /dev/sda6 /home ext4 rw,noatime,discard 0 0
>>>>                                     ^^^^^^^
>>>> I'd say that's your problem....
>>> Looks like the Sandisk U100 is a good SSD for me to put on my personal
>>> "avoid" list:
>>>
>>> http://thessdreview.com/our-reviews/asus-zenbook-ssd-review-not-necessarily-sandforce-driven-shows-significant-speed-bump/
>>>
>>> There are a number of SSD's which do not implement "trim" efficiently,
>>> so these days, the recommended way to use trim is to run the "fstrim"
>>> command out of crontab.
>> OK. Removing 'discard' made it much better (the 60-600 second
>> freezes are now 1-50 second freezes), but it's still at least an
>> order of magnitude worse than a normal HD. When writing, that is --
>> reading is very fast (when there's no writing going on).
>>
>> So, after reading up a bit on this trimming I'm thinking maybe my
>> filesystem's block sizes don't match up with my SSD's blocks (or
>> whatever its write unit is called). Then writing a FS block would
>> always write to multiple SSD blocks, causing multiple
>> read-erase-write sequences, right? So how can I check this, and how
>> can I make the FS blocks match the SSD blocks?
>    As Ted wrote, alignment isn't usually a problem with SSDs. And even if it
> was, it would be at most a factor 2 slow down and we don't seem to be at
> that fine grained level :)
>
> At this point you might try mounting the fs with nobarrier mount option (I
> know you tried that before but without discard the difference could be more
> visible), switching IO scheduler to CFQ (for crappy SSDs it actually isn't
> a bad choice), and we'll see how much we can squeeze out of your drive...

I repartitioned the drive and reinstalled ubuntu and after that it 
gladly wrote over 100 MB/s to the SSD without any hangs. However, after 
a couple of months I noticed it had degraded considerably, and it keeps 
degrading. Now it's slowly becoming completely unusable again, with 
write speeds of the magnitude 1 MB/s and dropping.

As far as I can tell I have not made any relevant changes. Also, the 
amount of free space hasn't changed considerably, but it seems that the 
longer it's been since I reformatted the drive the more free space is 
required for it to perform well.

So, maybe the cause is fragmentation? I tried running e4defrag and then 
fstrim, but it didn't really help (well, maybe a little bit, but after a 
couple of days it was back in unusable-land). Also, "e4defrag -c" gives 
a fragmenation score of less than 5, so...

Any ideas?


Best regards,
Marcus
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