RE: [RESEND] [PATCH] block: create ioctl to discard-or-zeroout a range of blocks

[Elliott, Robert (Server Storage)]

> -----Original Message-----
> From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Arnd Bergmann
> Sent: Thursday, 29 January, 2015 4:03 AM
> To: Darrick J. Wong
> Cc: Jens Axboe; Christoph Hellwig; linux-kernel@xxxxxxxxxxxxxxx; linux-
> fsdevel@xxxxxxxxxxxxxxx; linux-api@xxxxxxxxxxxxxxx; Jeff Layton; J. Bruce
> Fields
> Subject: Re: [RESEND] [PATCH] block: create ioctl to discard-or-zeroout a
> range of blocks
> 
> On Wednesday 28 January 2015 18:00:25 Darrick J. Wong wrote:
> > Create a new ioctl to expose the block layer's newfound ability to
> > issue either a zeroing discard, a WRITE SAME with a zero page, or a
> > regular write with the zero page.  This BLKZEROOUT2 ioctl takes
> > {start, length, flags} as parameters.  So far, the only flag available
> > is to enable the zeroing discard part -- without it, the call invokes
> > the old BLKZEROOUT behavior.  start and length have the same meaning
> > as in BLKZEROOUT.
> >
> > Furthermore, because BLKZEROOUT2 issues commands directly to the
> > storage device, we must invalidate the page cache (as a regular
> > O_DIRECT write would do) to avoid returning stale cache contents at a
> > later time.
> >
> > This patch depends on "block: Add discard flag to
> > blkdev_issue_zeroout() function" in Jens' for-3.20/core branch.
> >
> > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> >
> 
> Would this work ok for devices that fill discarded areas with all-ones
> instead of all-zeroes? I believe SD cards can do either.
> 
> 	Arnd

RAID software stacks need predictable data, and 0 is a lot easier to
deal with than 0xff in Galois field (XOR-based) math.

The SCSI and ATA standards both define bits to report if the device is
designed to return 0 after discard; they don't have a way to report
that any other particular non-zero value is returned.

---
Rob Elliott    HP Server Storage


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