linux-ext4 - Re: [PATCH 2/2] Add batched discard support for ext4.

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Date:	Tue, 20 Apr 2010 21:45:25 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Mark Lord <kernel@...savvy.com>
CC:	Greg Freemyer <greg.freemyer@...il.com>,
	Lukas Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, Jeff Moyer <jmoyer@...hat.com>,
	Edward Shishkin <eshishki@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>
Subject: Re: [PATCH 2/2] Add batched discard support for ext4.

Mark Lord wrote:
> On 20/04/10 05:21 PM, Greg Freemyer wrote:
>> Mark,
>>
>> This is the patch implementing the new discard logic.
> ..
>> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> ..
>>> +void ext4_trim_extent(struct super_block *sb, int start, int count,
>>> +               ext4_group_t group, struct ext4_buddy *e4b)
>>> +{
>>> +       ext4_fsblk_t discard_block;
>>> +       struct ext4_super_block *es = EXT4_SB(sb)->s_es;
>>> +       struct ext4_free_extent ex;
>>> +
>>> +       assert_spin_locked(ext4_group_lock_ptr(sb, group));
>>> +
>>> +       ex.fe_start = start;
>>> +       ex.fe_group = group;
>>> +       ex.fe_len = count;
>>> +
>>> +       mb_mark_used(e4b,&ex);
>>> +       ext4_unlock_group(sb, group);
>>> +
>>> +       discard_block = (ext4_fsblk_t)group *
>>> +                       EXT4_BLOCKS_PER_GROUP(sb)
>>> +                       + start
>>> +                       + le32_to_cpu(es->s_first_data_block);
>>> +       trace_ext4_discard_blocks(sb,
>>> +                       (unsigned long long)discard_block,
>>> +                       count);
>>> +       sb_issue_discard(sb, discard_block, count);
>>> +
>>> +       ext4_lock_group(sb, group);
>>> +       mb_free_blocks(NULL, e4b, start, ex.fe_len);
>>> +}
>>
>> Mark, unless I'm missing something, sb_issue_discard() above is going
>> to trigger a trim command for just the one range.  I thought the
>> benchmarks you did showed that a collection of ranges needed to be
>> built, then a single trim command invoked that trimmed that group of
>> ranges.
> ..
> 
> Mmm.. If that's what it is doing, then this patch set would be a
> complete disaster.
> It would take *hours* to do the initial TRIM.
> 
> Lukas ?

I'm confused; do we have an interface to send a trim command for multiple ranges?

I didn't think so ...  Lukas' patch is finding free ranges (above a size threshold)
to discard; it's not doing it a block at a time, if that's the concern.

-Eric
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