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

On Thu, Aug 13, 2009 at 2:28 PM, Greg Freemyer[off-list ref] wrote:

On Thu, Aug 13, 2009 at 4:44 PM, [off-list ref] wrote:

quoted

On Thu, 13 Aug 2009, Greg Freemyer wrote:

quoted

On Thu, Aug 13, 2009 at 12:33 PM, [off-list ref] wrote:

quoted

On Thu, 13 Aug 2009, Markus Trippelsdorf wrote:

quoted

On Thu, Aug 13, 2009 at 08:13:12AM -0700, Matthew Wilcox wrote:

quoted

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.

how does the block layer know what blocks are unused by the filesystem?

or would it be a case of the filesystem generating discard/trim requests to
the block layer so that it can maintain it's bitmap, and then the block
layer generating the requests to the drive below it?

David Lang

Yes, my thought.was that block layer would consume the discard/trim
requests from the filesystem in realtime to maintain the bitmap, then
at some later point in time when the system has extra resources it
would generate the calls down to the lower layers and eventually the
drive.

Why should the block layer be forced to maintain something that is
probably of use for only a limited number of cases? For example, the
devices I work on already maintain their own mapping of HOST-visible
LBAs to underlying storage, and I suspect that most such devices do.
So, you are duplicating something that we already do, and there is no
way that I am aware of to synchronise the two.

All we really need, I believe is for the UNMAP requests to come down
to us with writes barriered until we respond, and it is a relatively
cheap operation, although writes that are already in the cache and
uncommitted to disk present some issues if an UNMAP request comes down
for recently written blocks.

I highlight the lower layers because mdraid is also going to have to
be in the mix if raid5/6 is in use.  ie. At a minimum it will have to
adjust the block range to align with the stripe boundaries.

Greg
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-- 
Regards,
Richard Sharpe