Re: Sequential Scan Read-Ahead

Curt Sampson wrote:
> 3. Proof by testing. I wrote a little ruby program to seek to a
> random point in the first 2 GB of my raw disk partition and read
> 1-8 8K blocks of data. (This was done as one I/O request.) (Using
> the raw disk partition I avoid any filesystem buffering.) Here are
> typical results:
>
> 125 reads of 16x8K blocks: 1.9 sec, 66.04 req/sec. 15.1 ms/req, 0.946 ms/block
> 250 reads of 8x8K blocks: 1.9 sec, 132.3 req/sec. 7.56 ms/req, 0.945 ms/block
> 500 reads of 4x8K blocks: 2.5 sec, 199 req/sec. 5.03 ms/req, 1.26 ms/block
> 1000 reads of 2x8K blocks: 3.8 sec, 261.6 req/sec. 3.82 ms/req, 1.91 ms/block
> 2000 reads of 1x8K blocks: 6.4 sec, 310.4 req/sec. 3.22 ms/req, 3.22 ms/block
>
> The ratios of data retrieval speed per read for groups of adjacent
> 8K blocks, assuming a single 8K block reads in 1 time unit, are:
>
> 1 block 1.00
> 2 blocks 1.18
> 4 blocks 1.56
> 8 blocks 2.34
> 16 blocks 4.68

You mention you are reading from a raw partition. It is my
understanding that raw partition reads have no kernel read-ahead. (I
assume this because raw devices are devices, not file system files.) If
this is true, could we get numbers with kernel read-ahead active?

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