Fwd: Fwd: VFAT performance.

[Rogier Wolff]

Hi,

For the performance freaks: We're copying some data off a VFAT32
partition. We've opened the drive. (Yes I know, you're not supposed to
do that. "Don't do this at home folks!" :-)

When copying /dev/hda, we were able to achieve 11Mbyte per second: Our
100mpbs ethernet throughput. 

When copying large files off /mnt, we see a performance of about 7Mb
per second. We see the head seek to the FAT about twice per second. This
fits in with: 

	4K bytes of FAT contains 1024 fat entries. 
with a 4K clustersize, that would describe about 4Mbytes worth of data. 
So, at 7Mbytes per second we require a new FAT block twice per second. 

I think that we're loosing the 4Mbytes per second of performance due
to the 4 seeks per second that the drive has to perform. 

The way to fix this would be to be able to assign a higher cache
priority (*) to the blocks in the FAT, and to read more than just 4k
per seek to the FAT.

Just something to keep in mind when fiddling with the code again....

			Roger.

(*) i.e. expire them from the buffer cache less easily than normal
blocks.

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