Re: [PATCH 1/4] block: Allow devices to indicate whether discarded blocks are zeroed

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On Mon, Nov 23, 2009 at 11:37 AM, Ric Wheeler <rwheeler@xxxxxxxxxx> wrote:
> On 11/21/2009 09:43 PM, Mark Lord wrote:
>>
>> Matthew Wilcox wrote:
>>>
>>> On Sat, Nov 21, 2009 at 05:13:56AM -0500, Christoph Hellwig wrote:
>>>>
>>>> On Fri, Nov 20, 2009 at 09:45:21PM -0500, Martin K. Petersen wrote:
>>>>>
>>>>> The discard ioctl is used by mkfs utilities to clear a block device
>>>>> prior to putting metadata down.  However, not all devices return zeroed
>>>>> blocks after a discard.  Some drives return stale data, potentially
>>>>> containing old superblocks.  It is therefore important to know whether
>>>>> discarded blocks are properly zeroed.
>>>>
>>>> At least for mkfs.xfs we make sure to still zero the important areas
>>>> after the TRIM ioctl anyway.
>>>
>>> Could you change that to zero _before_ the TRIM?
>>
>> ..
>>
>>
>> Hopefully not for the drives that *don't* guarantee zeros after TRIM.
>> Eg. most existing ones, including all of the Indilinx chipset drives.
>>
>> Cheers
>>
>
> Note that writing to a block after a discard (write or trim) changes the
> state of that block back to allocated (mapped) in the target. Basically,
> that would seem to make the trim redundant and irrelevant.
>
> You can zero before the discard but that can still fail is the device
> returns garbage for a trim'ed block I think,
>
> Ric

I understood the mkfs process to be:

- discard / trim 100% of the block device
- write zeros to the relatively small group of blocks that the
filesystem assumes will be zero'd.

Seems like the only reasonable way to handle it.

Greg
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