linux-ext4 - Re: [PATCH] Set the initial TRIM information as TRIMMED

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Message-ID: <4ED8F566.4050005@redhat.com>
Date:	Fri, 02 Dec 2011 09:57:26 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Kyungmin Park <kmpark@...radead.org>
CC:	Lukas Czerner <lczerner@...hat.com>, tytso@....edu, tm@....ma,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Set the initial TRIM information as TRIMMED

On 12/1/11 6:01 PM, Kyungmin Park wrote:
> On 12/2/11, Eric Sandeen <sandeen@...hat.com> wrote:
>> On 12/1/11 1:00 AM, Kyungmin Park wrote:
>>> From: Kyungmin Park <kyungmin.park@...sung.com>
>>>
>>> Now trim information doesn't stored at disk so every boot time. it's
>>> cleared.
>>> and do the trim all disk groups.
>>> But assume that it's already trimmed at previous time so don't need to
>>> trim it again. So set the intial state as trimmed.
>>>
>>> Signed-off-by: Kyungmin Park <kyungmin.park@...sung.com>
>>> ---
>>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>>> index e2d8be8..97ef342 100644
>>> --- a/fs/ext4/mballoc.c
>>> +++ b/fs/ext4/mballoc.c
>>> @@ -1098,6 +1098,12 @@ int ext4_mb_init_group(struct super_block *sb,
>>> ext4_group_t group)
>>>  		goto err;
>>>  	}
>>>  	mark_page_accessed(page);
>>> +
>>> +	/*
>>> +	 * TRIM information is not stored at disk so set the initial
>>> +	 * state as trimmed. Since previous time it's already trimmed all
>>> +	 */
>>> +	EXT4_MB_GRP_SET_TRIMMED(this_grp);
>>
> Hi,
> 
>> Hm, so if there were freed but un-trimmed blocks at this point, we will
>> never trim them until we free _another_ block in the group, right?  That
>> might be a reasonable tradeoff, but it is somewhat surprising behavior.
>>
>> i.e. say we do:
>>
>> mount /mnt
>> rm -rf /mnt/very_big_file
>> umount /mnt
>>
>> mount /mnt
>> fitrim /mnt
> another word, you can run fitrim after rm -rf
> yes, it's trade-off.
> 
> In my case, phone scenario, no umount system and data partition. it's
> burden to trim at boot time. it has still slower boot time.
> some daemon or program run fitrm at filesystem. it consumes time and
> hurt other boot processes.
>>
>> then we won't trim anything at all, right, despite there being many
>> new free blocks?  Which would be rather unexpected.
>>
>> If we don't store the trimmed state on disk, I think we should
>> probably stick with the slower first-time trim, and the more obvious
>> behavior (all free blocks are always trimmed whenever a trim
>> command is issued).
> 
> Umm how do you think, introduce the trim force command for this?

Alternately, can we use a bit in bg_flags to keep a better view of this
state on disk, if this is critical?

-Eric

> Thank you,
> Kyungmin Park

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