This inadvertently went just to me, replying to all:

On Thu, Aug 13, 2009 at 8:58 PM, Richard
Sharpe[off-list ref] wrote:

On Thu, Aug 13, 2009 at 5:19 PM, Greg Freemyer[off-list ref] wrote:

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On Thu, Aug 13, 2009 at 6:20 PM, Richard
Sharpe[off-list ref] wrote:

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

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

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On Thu, 13 Aug 2009, Greg Freemyer wrote:

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

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On Thu, 13 Aug 2009, Markus Trippelsdorf wrote:

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

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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.

Richard,

Quoting the original email I saw in this thread:

quoted

The unfortunate thing about the TRIM command is that it's not NCQ, so
all NCQ commands have to finish, then we can send the TRIM command and
wait for it to finish, then we can send NCQ commands again.

So TRIM isn't free, and there's a better way for the drive to find
out that the contents of a block no longer matter -- write some new
data to it.  So if we just swapped a page in, and we're going to swap
something else back out again soon, just write it to the same location
instead of to a fresh location.  You've saved a command, and you've
saved the drive some work, plus you've allowed other users to continue
accessing the drive in the meantime.

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.

Thoughts on that are welcome.

quoted

My thought was that a bitmap was a better solution than a cache of
discard commands.

One of the biggest reasons is that a bitmap can coalesce the unused
areas into much larger discard ranges than a queue that will only have
a limited number of discards to coalesce.

OK, I misunderstood. For the work I did with an SSD company the UNMAP
requests were coming down as 1024 LBA DISCARDs/UNMAPs. If someone
deleted a multi-GB file that results in thousands of DISCARDS coming
down, which is a problem.

I think the ext4 implementation is sending down discards way smaller
than 1024 sectors.  Ted Tso posted something a few months ago that he
did a test and was seeing a massive number of them being sent from
ext4 to block.  The rest of the stack was not in place, so he did not
know the real performance impact.

However, I wonder if we cannot make do with merging in the block
layer, especially with XFS or Ext4.

That's the cache and coalesce approach, right?  Just a personal thing,
but we run
things like defrag in the background during off hours.

It seems to me that unmap is not all that different, why do we need to
do it even close in time proximity to the deletes?  With a bitmap, we
have total timing control of when the unmaps are forwarded down to the
device.  I like that timing control much better than a cache and
coalesce approach.

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And both Enterprise scsi and mdraid are desirous of larger discard ranges.

I also would like large discard ranges ... metadata updates in the
platform I am thinking of are transactional, and I would like to
reduce the number of transactions pushed through the metadata journal.

--
Regards,
Richard Sharpe

Greg