Messages in this thread | | | Date | Fri, 14 Aug 2009 18:03:10 -0400 | From | Mark Lord <> | Subject | Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed) |
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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@lang.hm> 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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