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Mon, 13 Jul 2020 13:45:01 +0000 (UTC) Date: Mon, 13 Jul 2020 15:45:00 +0200 From: Kevin Wolf To: Max Reitz Subject: Re: [PATCH for-5.1] file-posix: Mitigate file fragmentation with extent size hints Message-ID: <20200713134500.GC10318@linux.fritz.box> References: <20200707142329.48303-1-kwolf@redhat.com> <20200707161741.GG7002@linux.fritz.box> <451ed32d-72e8-0238-8793-f5a80ae991c4@redhat.com> <2974eb09-24fe-58b0-65d4-5cb550ed08f1@redhat.com> <20200713131243.GB10318@linux.fritz.box> MIME-Version: 1.0 In-Reply-To: <20200713131243.GB10318@linux.fritz.box> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="R3G7APHDIzY6R/pk" Content-Disposition: inline Received-SPF: pass client-ip=207.211.31.81; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/07/13 02:19:41 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no 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, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" --R3G7APHDIzY6R/pk Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Am 13.07.2020 um 15:12 hat Kevin Wolf geschrieben: > Am 13.07.2020 um 11:08 hat Max Reitz geschrieben: > > On 10.07.20 18:12, Max Reitz wrote: > > > On 07.07.20 18:17, Kevin Wolf wrote: > > >> Am 07.07.2020 um 16:23 hat Kevin Wolf geschrieben: > > >>> Espeically when O_DIRECT is used with image files so that the page = cache > > >>> indirection can't cause a merge of allocating requests, the file wi= ll > > >>> fragment on the file system layer, with a potentially very small > > >>> fragment size (this depends on the requests the guest sent). > > >>> > > >>> On Linux, fragmentation can be reduced by setting an extent size hi= nt > > >>> when creating the file (at least on XFS, it can't be set any more a= fter > > >>> the first extent has been allocated), basically giving raw files a > > >>> "cluster size" for allocation. > > >>> > > >>> This adds an create option to set the extent size hint, and changes= the > > >>> default from not setting a hint to setting it to 1 MB. The main rea= son > > >>> why qcow2 defaults to smaller cluster sizes is that COW becomes mor= e > > >>> expensive, which is not an issue with raw files, so we can choose a > > >>> larger file. The tradeoff here is only potentially wasted disk spac= e. > > >>> > > >>> For qcow2 (or other image formats) over file-posix, the advantage s= hould > > >>> even be greater because they grow sequentially without leaving hole= s, so > > >>> there won't be wasted space. Setting even larger extent size hints = for > > >>> such images may make sense. This can be done with the new option, b= ut > > >>> let's keep the default conservative for now. > > >>> > > >>> The effect is very visible with a test that intentionally creates a > > >>> badly fragmented file with qemu-img bench (the time difference whil= e > > >>> creating the file is already remarkable) and then looks at the numb= er of > > >>> extents and the take a simple "qemu-img map" takes. > > >>> > > >>> Without an extent size hint: > > >>> > > >>> $ ./qemu-img create -f raw -o extent_size_hint=3D0 ~/tmp/test.r= aw 10G > > >>> Formatting '/home/kwolf/tmp/test.raw', fmt=3Draw size=3D1073741= 8240 extent_size_hint=3D0 > > >>> $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 10000= 00 -S 8192 -o 0 > > >>> Sending 1000000 write requests, 4096 bytes each, 64 in parallel= (starting at offset 0, step size 8192) > > >>> Run completed in 25.848 seconds. > > >>> $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 10000= 00 -S 8192 -o 4096 > > >>> Sending 1000000 write requests, 4096 bytes each, 64 in parallel= (starting at offset 4096, step size 8192) > > >>> Run completed in 19.616 seconds. > > >>> $ filefrag ~/tmp/test.raw > > >>> /home/kwolf/tmp/test.raw: 2000000 extents found > > >>> $ time ./qemu-img map ~/tmp/test.raw > > >>> Offset Length Mapped to File > > >>> 0 0x1e8480000 0 /home/kwolf/tmp= /test.raw > > >>> > > >>> real 0m1,279s > > >>> user 0m0,043s > > >>> sys 0m1,226s > > >>> > > >>> With the new default extent size hint of 1 MB: > > >>> > > >>> $ ./qemu-img create -f raw -o extent_size_hint=3D1M ~/tmp/test.= raw 10G > > >>> Formatting '/home/kwolf/tmp/test.raw', fmt=3Draw size=3D1073741= 8240 extent_size_hint=3D1048576 > > >>> $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 10000= 00 -S 8192 -o 0 > > >>> Sending 1000000 write requests, 4096 bytes each, 64 in parallel= (starting at offset 0, step size 8192) > > >>> Run completed in 11.833 seconds. > > >>> $ ./qemu-img bench -f raw -t none -n -w ~/tmp/test.raw -c 10000= 00 -S 8192 -o 4096 > > >>> Sending 1000000 write requests, 4096 bytes each, 64 in parallel= (starting at offset 4096, step size 8192) > > >>> Run completed in 10.155 seconds. > > >>> $ filefrag ~/tmp/test.raw > > >>> /home/kwolf/tmp/test.raw: 178 extents found > > >>> $ time ./qemu-img map ~/tmp/test.raw > > >>> Offset Length Mapped to File > > >>> 0 0x1e8480000 0 /home/kwolf/tmp= /test.raw > > >>> > > >>> real 0m0,061s > > >>> user 0m0,040s > > >>> sys 0m0,014s > > >>> > > >>> Signed-off-by: Kevin Wolf > > >> > > >> I also need to squash in a few trivial qemu-iotests updates, for whi= ch I > > >> won't send a v2: > > >=20 > > > The additional specifications in 243 make it print a warning on tmpfs > > > (because the option doesn=E2=80=99t work there). I suppose the same = may be true > > > on other filesystems as well. Should it be filtered out? >=20 > I guess we just shouldn't print a warning if the requested hint is 0. >=20 > > This patch also breaks 059, 106, and 175. >=20 > Hm, I was sure I had tested raw... Anyway, 059 should filter out the > actual size (how could this ever work?), and 175 is obvious, too - it > tries to be clever, but not clever enough. >=20 > 106 is a bit mysterious because the error message implies that the > images end up smaller than before, which shouldn't be the case. I'll > have a look. Ah, it misinterprets MiB as KiB, so the error says the image is smaller than expected while it's actually larger. I'll just disable the extent size hint for this one, too. 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