linux-kernel - Re: 3.10: discard/trim support on md-raid1?

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Message-ID: <20130716171527.7c6cb722@notabene.brown>
Date:	Tue, 16 Jul 2013 17:15:27 +1000
From:	NeilBrown <neilb@...e.de>
To:	"Justin Piszcz" <jpiszcz@...idpixels.com>
Cc:	<linux-kernel@...r.kernel.org>, <linux-raid@...r.kernel.org>
Subject: Re: 3.10: discard/trim support on md-raid1?

On Sat, 13 Jul 2013 06:34:19 -0400 "Justin Piszcz" <jpiszcz@...idpixels.com>
wrote:

> Hello,
> 
> Running 3.10 and I see the following for an md-raid1 of two SSDs:
> 
> Checking /sys/block/md1/queue:
> add_random: 0
> discard_granularity: 512
> discard_max_bytes: 2147450880
> discard_zeroes_data: 0
> hw_sector_size: 512
> iostats: 0
> logical_block_size: 512
> max_hw_sectors_kb: 32767
> max_integrity_segments: 0
> max_sectors_kb: 512
> max_segment_size: 65536
> max_segments: 168
> minimum_io_size: 512
> nomerges: 0
> nr_requests: 128
> optimal_io_size: 0
> physical_block_size: 512
> read_ahead_kb: 8192
> rotational: 1
> rq_affinity: 0
> scheduler: none
> write_same_max_bytes: 0
> 
> What should be seen:
> rotational: 0

What has "rotational" got to do with "supports discard"?
There may be some correlation, but it isn't causal.

> And possibly:
> discard_zeroes_data: 1

This should be set as the 'or' of the same value from component devices.  And
does not enable or disable the use of discard.

I don't think that "does this device support discard" appears in sysfs.

I believe trim does work on md/raid1 if the underlying devices all support it.

NeilBrown

> 
> Can anyone confirm if there is a workaround to allow TRIM when using
> md-raid1?
> 
> Some related discussion here:
> http://us.generation-nt.com/answer/md-rotational-attribute-help-206571222.ht
> ml
> http://www.progtown.com/topic343938-ssd-strange-itself-conducts.html
> 
> 
> Justin.
> 
> 
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