Re: [PATCH] e2fsck: Discard free data and inode blocks.

[Eric Sandeen <sandeen@redhat.com>]

Ric Wheeler wrote:

...

>> Well, so far the only breakages I have seen was with lots of small TRIMs
>> (or UNMAPs, etc) issued in random pattern, never in case of mkfs which
>> is quite a opposite - big sequential ranges.
>>
>> Hangs should be covered by those two patches:
>>
>> http://marc.info/?l=linux-ext4&m=128774558623608&w=2
>> http://marc.info/?l=linux-ext4&m=128767099123375&w=2
>>
>> if, of course, they get upstream. Also there is a big win, when discard
>> also zeroes data, because in that case we can just skip inode table
>> initialization (zeroing) without any need of in-kernel lazyinit code
>> enabled. And we get all this for free. It was introduced with Sandeens
>> patch:
>>
>> http://marc.info/?l=linux-ext4&m=128234048208327&w=2
>>
>> So, I would rather leave it on by default.
>>
>> -Lukas
> 
> You cannot 100% depend on discard zeroing blocks - that is not a
> universal requirement of devices that support it. Specifically, for ATA
> devices, I think that there are optional bits that specify how a device
> will behave when you read from a trimmed region.

But don't we have the ability to test whether discard -does- zero blocks,
as advertised by the device?  And honestly if the device mis-reports, that
sounds like a device vendor problem to fix.

The proposal wasn't to discard and assume zero, but to check for that
behavior:

http://kerneltrap.org/mailarchive/linux-ext4/2010/9/21/6885628/thread

+		if (!retval && mke2fs_discard_zeroes_data(fs)) {
+			if (verbose)
+				printf(_("Discard succeeded and will return 0s "
+					 " - enabling lazy_itable_init\n"));
+			lazy_itable_init = 1;
+			lazy_itable_zeroed = 1;
+		}

so we're not depending on it zeroing blocks, we're just depending on it
advertising correctly whether or not it -does- zero.

-Eric

> Ric
>