From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: Raid 1 vs Raid 10 single thread performance Date: Thu, 11 Sep 2014 14:59:11 +1000 Message-ID: <20140911145911.47c0d857@notabene.brown> References: <20140911103110.42449c9e@notabene.brown> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; boundary="Sig_/aZL==FWMtrtcgz6BL2gnU1R"; protocol="application/pgp-signature" Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Bostjan Skufca Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids --Sig_/aZL==FWMtrtcgz6BL2gnU1R Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Thu, 11 Sep 2014 06:48:31 +0200 Bostjan Skufca wrote: > On 11 September 2014 02:31, NeilBrown wrote: > > On Wed, 10 Sep 2014 23:24:11 +0200 Bostjan Skufca wrot= e: > >> What does "properly" actually mean? > >> I was doing some benchmarks with various raid configurations and > >> figured out that the order of devices submitted to creation command is > >> significant. It also makes raid10 created in such mode reliable or > >> unreliable to a device failure (not partition failure, device failure, > >> which means that two raid underlying devices fail at once). > > > > I don't think you've really explained what "properly" means. How exact= ly do > > you get better throughput? > > > > If you want double-speed single-thread throughput on 2 devices, then cr= eate a > > 2-device RAID10 with "--layout=3Df2". >=20 > I went and retested a few things and I see I must have done something > wrong before: > - regardless whether I use --layout flag or not, and > - regardless of device cli arg order at array creation time, > =3D I always get double-speed single-thread throughput. Yaay! >=20 > Anyway, the thing is that regardless of -using -layout=3Df2 or not, > redundancy STILL depends on the order of command line arguments passed > to mdadm --create. > If I do: > - "sda1 sdb1 sda2 sdb2" - redundandcy is ok > - "sda1 sda2 sdb1 sdb2" - redundancy fails >=20 > Is there a flag that ensures redundancy in this particular case? > If not, don't you think the naive user (me, for example) would assume > that code is smart enough to ensure basic redundancy, if there are at > least two devices available? I cannot guess what other people will assume. I certainly cannot guard against all possible incorrect assumptions. If you create an array which doesn't have true redundancy you will get a message from the kernel saying: %s: WARNING: %s appears to be on the same physical disk as %s. True protection against single-disk failure might be compromised. Maybe mdadm could produce a similar message... >=20 > Because, if someone wants only performance and no redundancy, they > will look no further than raid 0. But raid10 strongly hints at > redundancy being incorporated in it. (I admit this is anecdotal, based > on my own experience and thought flow.) I really don't think there is any value is splitting a device into multiple partitions and putting more than one partition per device into an array. Have you tried using just one partition per device, making a RAID10 with --layout=3Df2 ?? NeilBrown >=20 >=20 > b. > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html --Sig_/aZL==FWMtrtcgz6BL2gnU1R Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.22 (GNU/Linux) iQIVAwUBVBEsHznsnt1WYoG5AQLl2Q//XYxFCt8oem4HzA/uANR9os9lGA1XBx6H l7yqh8CRvg32QHaxhh2an35PC40hhtutfrbeLbMQZsaZlWKzA/gMSFIm6MG/3ePw Z8J9vdCh/mCtWwo2Rpzwrj+iXRcXXmfp6cMMa/Ey+MWZbN7UpaICyUZzEYtPZ17w uLWWg64D7yNgF+W6lOqTOh3fbl2jPt4PjQZ6cc/AXWW2tByzUDM77KyOkvHNhviG wtzVfXr+oTbeCzeRbSg9Xk7iVhBzuw8nPKei18HvQ4VCP3pLNVTKqYISxlpQEPc4 U0qHWAHY8qHHDtE+gtY/tmxJlag72+LTO9S0KQpLX3YIaUFfYpQOMMg79VMXxkWR U14HjBGASk+BXsSVD8Xoosspv32yUOiql0hcHc7p8wKScqlA8ehCR9mlY437Iyhy Cc3eSxOCxY0ApfLuciVZcPhVOICoO0dQCkAz0OvwMITnHkJlMPVJMg9LlqvcqfD9 5jmvhBSFDBZTOlXziAcwiiiRIlyafd0VEK1f5g3CLZ60d/cL2DaL7jxOM6GEcVZD FLBggzDT94xavI5hyntRGkTKIFnybIjBTPIwaosez5E8zrdq1v2VFplDHki4PAHu 0d1ATXZsBLIQVONs22DAQE7tC90sCztRG6nRTd23LHiUt7a04nnyGKjxYj3E65yA CxmmAJpQsa8= =Aa7J -----END PGP SIGNATURE----- --Sig_/aZL==FWMtrtcgz6BL2gnU1R--