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SubjectRe: [PATCH, RFC] ext4: Use preallocation when reading from the inode table
Theodore Tso wrote:
> On Tue, Sep 23, 2008 at 08:18:54AM -0400, Ric Wheeler wrote:
>
>> I think that Alan is probably right - the magic number for modern drives
>> is probably closer to 256K. Having it be a /sys tunable (with a larger
>> default) would be a nice way to verify this.
>>
>
> I've played with this a bit, and with the "git status" workload,
> increasing the magic number beyond 16 (64k) doesn't actually help,
> because the number of inodes we need to touch wasn't big enough.
>
> So I switched to a different workload, which ran "find /path -size 0
> -print" with a much larger directory hierarchy. With that workload I
> got the following results:
>
> ra_bits ra_blocks ra_kb seconds % improvement
> 0 1 4 53.3 -
> 1 2 8 47.3 11.3%
> 2 4 16 41.7 21.8%
> 3 8 32 37.5 29.6%
> 4 16 64 34.4 35.5%
> 5 32 128 32 40.0%
> 6 64 256 30.7 42.4%
> 7 128 512 28.8 46.0%
> 8 256 1024 28.3 46.9%
> 9 512 2048 27.5 48.4%
>
> Given these numbers, I'm using a default of inode_readahead_bits of 5
> (i.3., 32 blocks, or 128k for 4k blocksize filesystems). For a
> workload that is 100% stat-based, without any I/O, it is possible to
> get better results by using a higher number, yes, but I'm concerned
> that a larger readahead may end up interfering with other reads. We
> need to run some other workloads to be sure a larger number won't
> cause problems before we go more aggressive on this parameter.
>
> I'll send the revised patch in another message.
>
> - Ted
>

That sounds about right for modern S-ATA/SAS drives. I would expect that
having this be a tunable knob might help for some types of storage (SSD
might not care, but should be faster in any case?).

ric



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