From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: md device io request split Date: Thu, 24 Nov 2011 06:30:51 +1100 Message-ID: <20111124063051.33cdad37@notabene.brown> References: <20111122093634.105520@gmx.net> <20111123133124.2042c1f4@notabene.brown> <20111123132216.162860@gmx.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=PGP-SHA1; boundary="Sig_/yOj.FKrBE/3vRxDlRer.=sn"; protocol="application/pgp-signature" Return-path: In-Reply-To: <20111123132216.162860@gmx.net> Sender: linux-raid-owner@vger.kernel.org To: Ramon =?ISO-8859-1?B?U2No9m5ib3Ju?= Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids --Sig_/yOj.FKrBE/3vRxDlRer.=sn Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On Wed, 23 Nov 2011 14:22:16 +0100 "Ramon Sch=F6nborn" wrote: > =20 > > RAID1 will only limit requests to 4K if the device beneath it is > > non-contiguous - e.g. a striped array or LVM arrangement were consecuti= ve > > blocks might be on different devices. >=20 > how does it know if a device is non-contiguous? Is there a way to have a = dm device "marked" like that or force md to use bigger requests? > Lets assume a host with about 20 raid1 devices consisting of dm-devices w= ith the mentioned overhead - do you think that not splitting the requests c= ould lead to a noticeable performance improvement? >=20 > Thanx for your help, > Ramon Sch=F6nborn If the device provide a "merge_bvec_fn", then it is assumed to not be contiguous. dm always sets this on its devices. I really have no idea what sort of overhead this creates. You would need to test it. I assume you are using dm simply as a partitioning tool with a single linear mapping per device. If this is the case it should be safe for testing to remove the line blk_queue_merge_bvec(md->queue, dm_merge_bvec); from drivers/md/dm.c and see how that change performance. If you have any = dm targets more simple than a single linear mapping with will almost certainly cause IO failure at some point so this should only be used for testing. NeilBrown --Sig_/yOj.FKrBE/3vRxDlRer.=sn Content-Type: application/pgp-signature; name=signature.asc Content-Disposition: attachment; filename=signature.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.18 (GNU/Linux) iQIVAwUBTs1J8Tnsnt1WYoG5AQJY3Q/+P+QVQlYCT2dIDKnrkDQ5SbMJI/UKW/JH isyMV7tYSxqQKCj0igcEHX9zz12T/W9FczZsdZnTlJCuwRN2WzVU90mFdc0XpESc aEAar3Ufd+8yB1qkMzc+k+tXtY4x83vWehu6AaAi+f4Bph7dMEptK8hkfSf42SQR oR+k45Fd3zBEpBdCcP9Of8bo4DCFn5cBrzKml07hQ3fLUUMeDAuNbK1NEZBZSPF4 6xE86QrnbNk3nJ5tMONoWHVB4sjMhc1BptANIf53EjVqE/RkIvhv1aYPMynzCnYI +aZU7fgP82FB8wvaEpN1cao1cqaOynaGUH/GgERLXxtO2RKaegBpwJGnsh8+1JBu OJnSGTGMCF57d1yLns92v/Gwkl2xQA1RlY1GGqp2Mk8C5iOJQCUzmSpFRcLckeUP wfjDWHDttHxYCn0Yrzh+Jd7pFmD5EVkN/j/hsAVIAzEypmRvHZpbkXzV9V1YKXVJ AnZyIhLbEO3u3z5P7qwpAMYa13DVj7l+YPJzHlCTQd14B7ceeVPX+Zcg/0kOvJTR pbVF8rZMxvX0fqpi+IMyJUviNDn296r3VOfbMx/V33jmy9bnbMriSKEEDRrRI9Ej Bu0/Xz8FJkt2bX9+pziy/6mJpLJG+DYM0kp6P8J6zHo+MpmwLBbfCpG3wKMwMjO3 hKR+9T6yr7A= =DgOn -----END PGP SIGNATURE----- --Sig_/yOj.FKrBE/3vRxDlRer.=sn--