On Mon, 22 Nov 2010, Ted Ts'o wrote:

quoted text
> On Thu, Oct 21, 2010 at 04:23:02PM +0200, Lukas Czerner wrote:
> > Since there are some slow SSD's out there and big thinly provisioned
> > storages on which it takes quite long to issue discard through whole
> > device, it would be nice to provide user the information about what is
> > going on and how long it will take (approximately).
> > 
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> 
> Hi Lukas,
> 
> I've looked at this patch, and one thing that disturbs me about it ---
> you are discarding the first percentage of the disk five percent times
> for no good reason just to get the timing, before then executing the
> discard for the entire disk.   There are a couple of problems with this:
> 
> *) For smart/competently implemented SSD's, discarding the same part
> of the disk five times might lead to a misleading timing --- the smart
> device could easily determine that the first 1% is already not in use
> after the first discard, and the subsequent 4 discards could be
> discard as no-ops.
> 
> *) Mark Lord has claimed that there exists a large number of
> incomptently implemented SSD's out there, that may actually be
> executing a flash erase of the discarded region.  If true, executing
> an extra flash erase on 1% of the disk for no good reason five times
> might not be the best thing to do for the longetivity of the device.
> 
> I was tempted to fix this up myself, but since I'm trying to get
> better at delegating work to others, may I suggest the following
> changes?
> 
> 1)  Implement block device ioctl's for the kernel that export the
> discard_granularity, discard_alignment, and max_discard_sectors.  
> 
> 2) Change mke2fs so that the discard is done in a separate function.
> Said function should attempt to fetch the discard_granularity,
> discard_alignment, and max_discard_sectors.
> 
> 3) This new function in mke2fs should start by discarding
> approximately 1% of device at a time, respecting discard_granularity
> and discard_alignment.  If the time to discard 1% of the device is
> less than a second, then it should double the amount that it discards
> at a time.  If the time to discard takes longer than 4 seconds, it
> should reduce the amount that it discards by half (again, always
> respecting discard_granularity and discard_alignment).  The function
> can display the amount of time elapsed and the estimated amount of
> time remaining after each chunk of the device that it discards,
> assuming it can use ^M to redraw the progress report (which of course
> should be suppressed if the -q option is specified on the command
> line).
> 
> 
> This design doesn't "waste" any discards, which is both faster and
> reduces wear on badly designed SSD's.  It also continuously updates
> the user with the amount of time it takes to complete the discard
> process.  It also will respect the discard_granularity and
> discard_alignment restrictions; and of course, it allows the user to
> interrupt the discard, without needing a special kernel patch.
> 
> Does this make sense to you?
> 
> 						- Ted
> 

Hi Ted,

this absolutely make sense to me. I like the idea way better than what I
had done in my patch (actually I was probably lazy to do it right in the
first place:)). So, I'll add this into my todo list and hopefully find
some time to deal with it ASAP.

Thanks for suggestions!

-Lukas

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