From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick Mansfield Subject: Re: [RFC] Multi-path IO in 2.5/2.6 ? Date: Tue, 10 Sep 2002 09:20:15 -0700 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <20020910092015.B7765@eng2.beaverton.ibm.com> References: <200209091734.g89HY5p11796@localhost.localdomain> <20020909170847.A24352@eng2.beaverton.ibm.com> <10209100055.ZM65139@classic.engr.sgi.com> <20020910130427.GP2992@marowsky-bree.de> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20020910130427.GP2992@marowsky-bree.de>; from lmb@suse.de on Tue, Sep 10, 2002 at 03:04:27PM +0200 List-Id: linux-scsi@vger.kernel.org To: Lars Marowsky-Bree Cc: linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org On Tue, Sep 10, 2002 at 03:04:27PM +0200, Lars Marowsky-Bree wrote: > On 2002-09-10T00:55:58, > Jeremy Higdon said: >=20 > > Is there any plan to do something for hardware RAIDs in which two d= ifferent > > RAID controllers can get to the same logical unit, but you pay a pe= rformance > > penalty when you access the lun via both controllers?=20 >=20 > This is implemented in the md multipath patch in 2.4; it distinguishe= s between > "active" and "spare" paths. >=20 > The LVM1 patch also does this by having priorities for each path and = only > going to the next priority group if all paths in the current one have= failed, > which IMHO is slightly over the top but there is always someone who m= ight need > it ;-) >=20 > This functionality is a generic requirement IMHO. >=20 >=20 > Sincerely, > Lars Marowsky-Br=E9e The current scsi multi-path has a default setting of "last path used", = so that it will always work with such controllers (I refer to such control= lers as fail-over devices). You have to modify the config, or boot with a flag to get round-robin path selection. Right now, all paths will start out on the same adapter (initiator), this is bad if you have more than a few arrays attached to and adapter. I was planning on implementing something similiar to what you describe=20 for LVM1, with path weighting. Yes, it seems a bit heavy, but if there are only two weights or priorities, it looks just like your active/spar= e code, and is a bit more flexible. =46iguring out what ports are active or spare is not easy, and varies from array to array. This is a (another) good canidate for user level probing/configuration. I will likely probably hard-code device personallity information into the kernel for now, and hopefully in 2.7 we could move SCSI to user level probe/scan/configure. I have heard that some arrays at one time had a small penalty for switching controllers, and it was best to use the last path used, but it was still OK to use both at the same time (cache warmth). I was trying to think of a way to allow good load balancing in such cases, but didn't come up with a good solution. Like use last path used selection, and once a path is too "busy" move to another path - but then all devices might switch at the same time; some hearistics or timing could probably be added to avoid such problems. The code allows for path selection in a single function, so it should not be difficult to add more complex path selection. I have also put NUMA path selection into the latest code. I've tested it with 2.5.32, and a bunch of NUMA + NUMAQ patches on IBM/Sequent NUMAQ systems. -- Patrick Mansfield - To unsubscribe from this list: send the line "unsubscribe linux-scsi" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html