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

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Date:	Tue, 26 Feb 2013 20:41:36 +0200
From:	Marcus Sundman <marcus@...ox.fi>
To:	Theodore Ts'o <tytso@....edu>, Dave Chinner <david@...morbit.com>,
	Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: Debugging system freezes on filesystem writes

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?

Best regards,
Marcus

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