linux-ext4 - Re: "data=writeback" and TRIM don't get along

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Date:	Thu, 08 Apr 2010 13:48:51 +0200
From:	Nebojsa Trpkovic <trx.lists@...il.com>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	linux-ext4@...r.kernel.org
Subject: Re: "data=writeback" and TRIM don't get along

On 04/08/10 06:47, Eric Sandeen wrote:
> Eric Sandeen wrote:
>> Eric Sandeen wrote:
>>> I'll have to think about the right way to do this... it seems pretty
>>> convoluted to me right now.
>>>   
>> Something like this probably works, but I really REALLY would not test
>> it on an important filesystem.   :)
>>
>> I'm not sure it's a good idea to discard it before returning it
>> to the prealloc pool, because it may well get re-used again
>> quickly.... not sure if that's helpful.
> 
> (now I'm really talking to myself, but scratch that bit - 
> ext4_mb_return_to_preallocation is pretty much a no-op)
> 
> -Eric
> 
>> Just a note, I think eventually we may move to more of a batch discard
>> in the background, because these little discards are actually quite
>> inefficient on the hardware we've tested so far.
>>
>> -Eric
>>
>> p.s. really.  Don't test this with important data.  I haven't tested
>> it at all yet.
>>
>> Index: linux-2.6/fs/ext4/mballoc.c
>> ===================================================================
>> --- linux-2.6.orig/fs/ext4/mballoc.c
>> +++ linux-2.6/fs/ext4/mballoc.c
>> @@ -4602,6 +4606,8 @@ do_more:
>>  		mb_clear_bits(bitmap_bh->b_data, bit, count);
>>  		ext4_mb_free_metadata(handle, &e4b, new_entry);
>>  	} else {
>> +		ext4_fsblk_t discard_block;
>> +
>>  		/* need to update group_info->bb_free and bitmap
>>  		 * with group lock held. generate_buddy look at
>>  		 * them with group lock_held
>> @@ -4609,6 +4615,11 @@ do_more:
>>  		ext4_lock_group(sb, block_group);
>>  		mb_clear_bits(bitmap_bh->b_data, bit, count);
>>  		mb_free_blocks(inode, &e4b, bit, count);
>> +		discard_block = bit +
>> +				ext4_group_first_block_no(sb, block_group);
>> +		trace_ext4_discard_blocks(sb,
>> +				(unsigned long long)discard_block, count);
>> +		sb_issue_discard(sb, discard_block, count);
>>  		ext4_mb_return_to_preallocation(inode, &e4b, block, count);
>>  	}

Well, to be honest, I'm not some programmer guy, so I doubt my skills
can be of any help here.

Second, unfortunately, my SSD is now my root partition (just one big
sda1), so I cannot experiment with it too much.

And finally,
>> I'm not sure it's a good idea to discard it before returning it
>> to the prealloc pool, because it may well get re-used again
>> quickly.... not sure if that's helpful.

I'm not sure I understood you well about this prealloc pool - re-using
mechanism, but...
AFAIK, modern SSDs are using very aggressive wear-leveling algorithms.
Writing two times into the same filesystem sector almost newer goes to
the same hardware sector. Therefore, saving sectors from discarding and
for re-using makes no much sense - once re-used it will be written to
some other physical NAND memory cell anyways. Guess we can discard it as
soon as we have no valid data in it.

Nebojsa
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