Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

[Mark Lord]

James Bottomley wrote:
> On Thu, 2009-08-13 at 14:15 -0400, Greg Freemyer wrote:
> > On Thu, Aug 13, 2009 at 12:33 PM, <david@xxxxxxx> wrote:
> > > On Thu, 13 Aug 2009, Markus Trippelsdorf wrote:
> > >
> > > > On Thu, Aug 13, 2009 at 08:13:12AM -0700, Matthew Wilcox wrote:
> > > > > I am planning a complete overhaul of the discard work.  Users can send
> > > > > down discard requests as frequently as they like.  The block layer will
> > > > > cache them, and invalidate them if writes come through.  Periodically,
> > > > > the block layer will send down a TRIM or an UNMAP (depending on the
> > > > > underlying device) and get rid of the blocks that have remained unwanted
> > > > > in the interim.
> > > > That is a very good idea. I've tested your original TRIM implementation on
> > > > my Vertex yesterday and it was awful ;-). The SSD needs hundreds of
> > > > milliseconds to digest a single TRIM command. And since your
> > > > implementation
> > > > sends a TRIM for each extent of each deleted file, the whole system is
> > > > unusable after a short while.
> > > > An optimal solution would be to consolidate the discard requests, bundle
> > > > them and send them to the drive as infrequent as possible.
> > > or queue them up and send them when the drive is idle (you would need to
> > > keep track to make sure the space isn't re-used)
> > >
> > > as an example, if you would consider spinning down a drive you don't hurt
> > > performance by sending accumulated trim commands.
> > >
> > > David Lang
> > An alternate approach is the block layer maintain its own bitmap of
> > used unused sectors / blocks. Unmap commands from the filesystem just
> > cause the bitmap to be updated.  No other effect.
> >
> > (Big unknown: Where will the bitmap live between reboots?  Require DM
> > volumes so we can have a dedicated bitmap volume in the mix to store
> > the bitmap to? Maybe on mount, the filesystem has to be scanned to
> > initially populate the bitmap?   Other options?)
>
> I wouldn't really have it live anywhere.  Discard is best effort; it's
> not required for fs integrity.  As long as we don't discard an in-use
> block we're free to do anything else (including forget to discard,
> rediscard a discarded block etc).
>
> It is theoretically possible to run all of this from user space using
> the fs mappings, a bit like a defrag command.
..

Already a work-in-progress -- see my wiper.sh script on the hdparm page
at sourceforge.  Trimming 50+GB of free space on a 120GB Vertex
(over 100 million sectors) takes a *single* TRIM command,
and completes in only a couple of seconds.

Cheers
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