Re: [PATCH 1/2 v3] EXT4: Secure Delete: Zero out file data

[Allison Henderson <achender@linux.vnet.ibm.com>]

On 06/30/2011 06:18 PM, Martin K. Petersen wrote:
>>>>>> "Allison" == Allison Henderson<achender@linux.vnet.ibm.com>  writes:
>
>>> Does sb_issue_zeroout() use the SCSI "write same" feature in the
>>> background?  That would avoid busying the CPU/controller/bus with
>>> writing out zeroes, which might be expensive for a large file.
>>>
> Allison>  Hmm, that's a good question, I will dig into it and see if I
> Allison>  can find out.
>
> This is a bit of an ongoing project.
>
> Unfortunately WRITE SAME is quirk central as many drives only implement
> block ranges corresponding to what RAID vendors told them they needed.
> Many of the drives I tested have internal caps at 16 or 32MB and will
> fail in interesting (i.e. not necessarily graceful) ways if given bigger
> ranges.
>
> I've been lobbying for a way for devices to report their WRITE SAME
> limit for a while. That feature finally made it into the latest SBC3
> draft. The changes required to support it went into the SCSI layer
> during the last merge window. What remains is wiring this up to
> blkdev_issue_zeroout(). I have some patches sitting in my queue that I
> hope to get polished and submitted soon.
>
> Anyway. From a filesystem perspective sb_issue_zeroout() interface is
> definitely the way to go. WRITE SAME will eventually be called if the
> device supports it.
>
Ah, I see, well that answers that question.  Thx for the through 
explanation.  :)