linux-kernel - Re: BLKZEROOUT not zeroing md dev on VMDK

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Date:	Fri, 27 May 2016 05:45:46 +0100
From:	Sitsofe Wheeler <sitsofe@...il.com>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	Shaohua Li <shli@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Arvind Kumar <arvindkumar@...are.com>,
	VMware PV-Drivers <pv-drivers@...are.com>,
	linux-raid@...r.kernel.org,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	linux-block@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: BLKZEROOUT not zeroing md dev on VMDK

On 27 May 2016 at 05:18, Darrick J. Wong <darrick.wong@...cle.com> wrote:
>
> It's possible that the pvscsi device advertised WRITE SAME, but if the device
> sends back ILLEGAL REQUEST then the SCSI disk driver will set
> write_same_max_bytes=0.  Subsequent BLKZEROOUT attempts will then issue writes
> of zeroes to the drive.

Thanks for following up on this but that's not what happens on the md
device - you can go on to issue as many BLKZEROOUT requests as you
like but the md disk is never zeroed nor is an error returned.

I filed a bug at https://bugzilla.kernel.org/show_bug.cgi?id=118581
(see https://bugzilla.kernel.org/show_bug.cgi?id=118581#c6 for
alternative reproduction steps that use scsi_debug and can be reworked
to impact device mapper) and Shaohua Li noted that
blkdev_issue_write_same could return 0 even when the disk didn't
support write same (see
https://bugzilla.kernel.org/show_bug.cgi?id=118581#c8 ).

Shaohua went on to create a patch for this ("block: correctly fallback
for zeroout" - https://patchwork.kernel.org/patch/9137311/ ) which has
yet to be reviewed.

-- 
Sitsofe | http://sucs.org/~sits/