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Date:	Tue, 05 Apr 2011 09:39:07 -0700
From:	Eric Sandeen <sandeen@...hat.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	ext4 development <linux-ext4@...r.kernel.org>,
	Zeev Tarantov <zeev.tarantov@...il.com>
Subject: Re: [PATCH] e2fsprogs: don't set stripe/stride to 1 block in mkfs

On 4/5/11 1:10 AM, Andreas Dilger wrote:
> On 2011-04-04, at 9:11 AM, Eric Sandeen wrote:
>> Block devices may set minimum or optimal IO hints equal to
>> blocksize; in this case there is really nothing for ext4
>> to do with this information (i.e. search for a block-aligned
>> allocation?) so don't set fs geometry with single-block
>> values.
>>
>> Zeev also reported that with a block-sized stripe, the
>> ext4 allocator spends time spinning in ext4_mb_scan_aligned(),
>> oddly enough.
>>
>> Reported-by: Zeev Tarantov <zeev.tarantov@...il.com>
>> Signed-off-by: Eric Sandeen <sandeen@...hat.com>
>> ---
>>
>> diff --git a/misc/mke2fs.c b/misc/mke2fs.c
>> index 9798b88..74b838c 100644
>> --- a/misc/mke2fs.c
>> +++ b/misc/mke2fs.c
>> @@ -1135,8 +1135,11 @@ static int get_device_geometry(const char *file,
>> 	if ((opt_io == 0) && (psector_size > blocksize))
>> 		opt_io = psector_size;
>>
>> -	fs_param->s_raid_stride = min_io / blocksize;
>> -	fs_param->s_raid_stripe_width = opt_io / blocksize;
>> +	/* setting stripe/stride to blocksize is pointless */
>> +	if (min_io > blocksize)
>> +		fs_param->s_raid_stride = min_io / blocksize;
>> +	if (opt_io > blocksize)
>> +		fs_param->s_raid_stripe_width = opt_io / blocksize;
> 
> I don't think it is harmful to specify an mballoc alignment that is
> an even multiple of the underlying device IO size (e.g. at least
> 256kB or 512kB).
> 
> If the underlying device (e.g. zram) is reporting 16kB or 64kB opt_io
> size because that is PAGE_SIZE, but blocksize is 4kB, then we will
> have the same performance problem again.> 
> Cheers, Andreas

I need to look into why ext4_mb_scan_aligned is so inefficient for a block-sized stripe.

In practice I don't think we've seen this problem with stripe size at 4 or 8 or 16 blocks; it may just be less apparent.  I think the function steps through by stripe-sized units, and if that is 1 block, it's a lot of stepping.  

        while (i < EXT4_BLOCKS_PER_GROUP(sb)) {
...
                if (!mb_test_bit(i, bitmap)) {
...
                }
                i += sbi->s_stripe;
        }

But in any case, setting stripe alignment to 1 block makes no sense to me, and I see no reason to do it at mkfs time...

-Eric
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