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envelope-from=LMa@suse.com; helo=m9a0014g.houston.softwaregrp.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/03 03:07:51 X-ACL-Warn: Detected OS = Linux 3.11 and newer [fuzzy] X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, HTML_MESSAGE=0.001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001, UNPARSEABLE_RELAY=0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "qemu-devel@nongnu.org" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --_000_BY5PR18MB33139BC1147B5E1224A08874C5880BY5PR18MB3313namp_ Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Hi Kevin, Thanks for the explanation! Lin ________________________________ From: Kevin Wolf Sent: Tuesday, June 2, 2020 7:06 PM To: Lin Ma Cc: qemu-devel@nongnu.org ; pbonzini@redhat.com Subject: Re: [question] Partial sector issue while discard in qcow2 image Am 02.06.2020 um 09:45 hat Lin Ma geschrieben: > Hi all, > > During woring to add GET LBA STATUS support in qemu scsi emulation > layer, I encountered an unmap issue with qcow2 image, It's likely > about how to unmap partial clusters. e.g.: > > With these default values: > * the default value of s->qdev.blocksize: 512 > * the default value of s->cluster_size of qcow2 image: 65536 > > Running 'sg_unmap -l 1024 -n 32 /dev/sda' hits the condition > 'if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size))' in the > function qcow2_co_pdiscard, Then it won't perform > qcow2_cluster_discard(), So the corresponding clusters won't be > discard in this situation, Correct? Yes, this is correct. Discard is just a hint, so doing nothing is a perfectly valid implementation. In the case of qcow2, only full clusters are discarded. > Of cause, with the default blocksize and cluster_size, The below > examples won't hit the condition 'if (!QEMU_IS_ALIGNED...'. > sg_unmap -l 1024 -n 128 /dev/sda > or > sg_unmap -l 256 -n 128 /dev/sda Yes, and when discarding whole block devices (e.g. while creating a new filesystem on them) or large files, you'll probably get this case for most parts. Kevin --_000_BY5PR18MB33139BC1147B5E1224A08874C5880BY5PR18MB3313namp_ Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Hi Kevin,

Thanks for the explanation!

Lin

From: Kevin Wolf <kwol= f@redhat.com>
Sent: Tuesday, June 2, 2020 7:06 PM
To: Lin Ma <LMa@suse.com>
Cc: qemu-devel@nongnu.org <qemu-devel@nongnu.org>; pbonzini@re= dhat.com <pbonzini@redhat.com>
Subject: Re: [question] Partial sector issue while discard in qcow2 = image
 
Am 02.06.2020 um 09:45 hat Lin Ma geschrieben:
> Hi all,
>
> During  woring to add GET LBA STATUS support in qemu scsi emulati= on
> layer, I encountered an unmap issue with qcow2 image, It's likely
> about how to unmap partial clusters. e.g.:
>
> With these default values:
> * the default value of s->qdev.blocksize: 512
> * the default value of s->cluster_size of qcow2 image: 65536
>
> Running 'sg_unmap -l 1024 -n 32 /dev/sda' hits the condition
> 'if (!QEMU_IS_ALIGNED(offset | bytes, s->cluster_size))' in the
> function qcow2_co_pdiscard, Then it won't perform
> qcow2_cluster_discard(), So the corresponding clusters won't be
> discard in this situation, Correct?

Yes, this is correct. Discard is just a hint, so doing nothing is a
perfectly valid implementation. In the case of qcow2, only full clusters are discarded.

> Of cause, with the default blocksize and cluster_size, The below
> examples won't hit the condition 'if (!QEMU_IS_ALIGNED...'.
> sg_unmap -l 1024 -n 128 /dev/sda
> or
> sg_unmap -l 256 -n 128 /dev/sda

Yes, and when discarding whole block devices (e.g. while creating a new
filesystem on them) or large files, you'll probably get this case for
most parts.

Kevin

--_000_BY5PR18MB33139BC1147B5E1224A08874C5880BY5PR18MB3313namp_--