fast compressed fs (was Re: -nice tree)

[Pavel Machek <pavel@suse.cz>]

Hi!

> > How?
> 
> Run two parallel tasks: 1. write pages to disk by queuing the I/Os;
> 2. compress unqueued pages.
> 
> The first task won't use much CPU because it's always waiting for disk
> DMAs to complete.  While it sleeps, the second task runs.
> Alternatively this can be implemented using polling for I/O
> completions in the second task, if that's easier.
> 
> The first task should keep the I/O queue full enough to sustain
> writing, but not much fuller than that.  Either a fixed queue length
> will be fine, or it is easy to adjust the queue length dynamically by
> enlarging it if any I/O completions occur when the queue is empty.
> 
> The second task consumes uncompressed pages and makes available
> compressed pages.
> 
> When the first task wants to queue more pages for I/O, it first checks
> the compressed-page list.  If there are any, they are queued,
> otherwise it consumes uncompressed pages.
> 
> This will automatically converge on an optimal balance between
> compressed and uncompressed page writing, provided the disk is using
> DMA, which they do on all modern system.
> 
> This is actually better than fully enabling or disabling compression.

Very clever. This even could be used for a filesystem to make writes faster...
Hmm, compressed fs and *faster* than normal... sounds good.

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