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

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Date:	Tue, 16 Jul 2013 01:22:28 -0700 (PDT)
From:	David Lang <david@...g.hm>
To:	Justin Piszcz <jpiszcz@...idpixels.com>
cc:	NeilBrown <neilb@...e.de>,
	open list <linux-kernel@...r.kernel.org>,
	linux-raid@...r.kernel.org
Subject: Re: 3.10: discard/trim support on md-raid1?

Part of this depends on the exact failure mode. I've seen cases where drives 
fail, and the drive does a bunch of retries, then the OS does a bunch of 
retries, and eventually the read fails, but in the meantime, everything stalls 
for a long time.

I've even seen the same thing in at least one case where there was a hardware 
RAID card in use.

David Lang

On Tue, 16 Jul 2013, Justin Piszcz wrote:

> Thanks for the replies,
>
> After some further testing..
> When I ran a repair on the md's sync_action, the system would reduce
> I/O to the RAID-1 to 14kb/s or even less when it hit a certain number
> of blocks and effectively locked the system every time.
> It turned out to be a bad SSD (it also failed Intel's Secure Erase), I RMA'd it.
> Interesting though that it did not drop out of the array but froze the
> system (the failure scenario was odd).
>
> Justin.
>
>
>
> On Tue, Jul 16, 2013 at 3:15 AM, NeilBrown <neilb@...e.de> wrote:
>> 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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>>
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