[OpenAFS-devel] ZFS cache usage tracking

[Niklas Edmundsson]

On Fri, 4 Sep 2009, Andrew Deason wrote:

<snip>
> The discrepancy we're hitting can be easily seen by plainly doing this
> on a ZFS filesystem with default settings:
>
> dd if=/dev/urandom of=somefile bs=1024 count=1024
> sleep 10
> dd if=/dev/urandom of=somefile bs=1 count=1
> sleep 10
> stat somefile | grep Size
>  Size: 1               Blocks: 261        IO Block: 131072 regular file
>
> So, a file that is 1M then truncated down to 1 byte still takes up
> 130k-ish disk space.
<snap>
> I'm not sure what to do about this. Does anyone reading this know enough
> about ZFS internals to shed some light on this? I've got a few potential
> directions to go in, though:
<snappeti>

Some additional (probably stupid) thoughts:

- What happens if you truncate the file by opening it with O_TRUNC, or
   are we doing that already, or will it make the cache go bezerk?

- I was going to suggest doing fstat() on the fd, but it
   probably won't help as it would return blocks _on disk_, and stuff
   still hanging in memory pending a disk flush won't be counted.

- Or just use memcache...

/Nikke
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