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Message-ID: <BANLkTimHMM6gNcFFP62eU-SUfaCPyhWE_A@mail.gmail.com> Date: Thu, 16 Jun 2011 20:05:03 +0200 From: Kay Sievers <kay.sievers@...y.org> To: dgilbert@...erlog.com Cc: James Bottomley <James.Bottomley@...senpartnership.com>, Greg KH <greg@...ah.com>, Nao Nishijima <nao.nishijima.xt@...achi.com>, linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org, jcm@...hat.com, hare@...e.de, stefanr@...6.in-berlin.de, yrl.pp-manager.tt@...achi.com Subject: Re: [PATCH 1/3] [RFC] genhd: add a new attribute in device structure On Thu, Jun 16, 2011 at 20:00, Douglas Gilbert <dgilbert@...erlog.com> wrote: > On 11-06-16 01:20 PM, Kay Sievers wrote: >> On Thu, Jun 16, 2011 at 19:09, Kay Sievers<kay.sievers@...y.org> wrote: >>> On Thu, Jun 16, 2011 at 18:25, James Bottomley >>> <James.Bottomley@...senpartnership.com> wrote: >>>> >>>> On Thu, 2011-06-16 at 09:14 -0700, Greg KH wrote: >>>>> >>>>> All userspace naming will be taken care of by the usual udev rules, so >>>>>> >>>>>> for disks, something like /dev/disk/by-preferred/<fred> which would >>>>>> be >>>>>> the usual symbolic link. >>>>> >>>>> No, udev can not create such a link after the preferred name is set, as >>>>> it has no way of knowing that the name was set. >>>> >>>> It can if we trigger a uevent. Note: I'm not advocating this ... I'd be >>>> equally happy having whatever sets the kernel name create the link (or >>>> tickle udev to create it). We definitely require device links, though, >>>> to get this to work. >> >> Guess all that would work now, including mount(8) not canonicalizing. >> What would happen if we mount: >> /dev/disk/by-pretty/foo >> and some tool later thinks the pretty name should better be 'bar', it >> writes the name to /sys, we get a uevent, the old link disappears, we >> get a new link, mount has no device node anymore for the mounted >> device ... >> >> So we basically get a one-shot additional pretty name? Guess, the >> _single_ name changed anytime later just asks for serious problems. We >> need to set it very early to be really useful, but how, where is it >> coming from? > > One obvious candidate for a preferred block device name > is: > - a SATA disk's WWN (NAA 5 64 bit), or > - a SCSI disk's logical unit name (e.g. SAS: NAA 5) > > These names (actually numbers) are meant to be world wide > unique. > > The kernel's device naming (following from how devices are > discovered) is topological. However at higher levels > the user is interested in the device identity. So if > unique device names were used as preferred names and > preferred names were unique (in a Linux system at any > given time) then any subsequent path to an existing device > would be highlighted. [That is because subsequent attempts > to create its preferred name would fail because it is > already there.] > > You don't need thousands of dollars of equipment to > demonstrate this point. An external single disk > SATA enclosure with a USB and eSATA interface will do. Udev does that already since quite a while. This is my cheap laptop: # find /dev/disk/ -name "wwn*" /dev/disk/by-id/wwn-0x50015179593f3038-part1 /dev/disk/by-id/wwn-0x50015179593f3038-part4 /dev/disk/by-id/wwn-0x50015179593f3038-part3 /dev/disk/by-id/wwn-0x50015179593f3038-part2 /dev/disk/by-id/wwn-0x50015179593f3038 Kay -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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