The three different names ZFS stores for each vdev disk (on Illumos)

August 16, 2017

I sort of mentioned yesterday that ZFS keeps information on several different ways of identifying disks in pools. To be specific, it keeps three different names or ways of identifying each disk. You can see this with 'zdb -C' on a pool, so here's a representative sample:

# zdb -C rpool
MOS Configuration:
    type: 'disk'
    id: 0
    guid: 15557853432972548123
    path: '/dev/dsk/c3t0d0s0'
    devid: 'id1,sd@SATA_____INTEL_SSDSC2BB08__BTWL4114016X080KGN/a'
    phys_path: '/pci@0,0/pci15d9,714@1f,2/disk@0,0:a'

The guid is ZFS's internal identifier for the disk, and is stored on the disk itself as part of the disk label. Since you have to find the disk to read it, it's not something that ZFS uses to find disks, although it is part of verifying that ZFS has found the right one. The three actual names for the disk are reported here as path, devid aka 'device id', and phys_path aka 'physical path'.

The path is straightforward; it's the filesystem path to the device, which here is a conventional OmniOS (Illumos, Solaris) cNtNdNsN name typical of a plain, non-multipathed disk. As this is a directly attached SATA disk, the phys_path shows us the PCI information about the controller for the disk in the form of a PCI device name. If we pulled this disk and replaced it with a new one, both of those would stay the same, since with a directly attached disk they're based on physical topology and neither has changed. However, the devid is clearly based on the disks's identity information; it has the vendor name, the 'product id', and the serial number (as returned by the disk itself in response to SATA inquiry commands). This will be the same more or less regardless of where the disk is connected to the system, so ZFS (and anything else) can find the disk wherever it is.

(I believe that the 'id1,sd@' portion of the devid is simply giving us a namespace for the rest of it. See 'prtconf -v' for another representation of all of this information and much more.)

Multipathed disks (such as the iSCSI disks on our fileservers) look somewhat different. For them, the filesystem device name (and thus path) looks like c5t<long identifier>d0s0, the physical path is /scsivhci/disk@g<long identifier>, and the devid_ is not particularly useful in finding the specific physical disk because our iSCSI targets generate synthetic disk 'serial numbers' based on their slot position (and the target's hostname, which at least lets me see which target a particular OmniOS-level multipathed disk is supposed to be coming from). As it happens, I already know that OmniOS multipathing identifies disks only by their device ids, so all three names are functionally the same thing, just expressed in different forms.

If you remove a disk entirely, all three of these names go away for both plain directly attached disks and multipath disks. If you replace a plain disk with a new or different one, the filesystem path and physical path will normally still work but the devid of the old disk is gone; ZFS can open the disk but will report that it has a missing or corrupt label. If you replace a multipathed disk with a new one and the true disk serial number is visible to OmniOS, all of the old names go away since they're all (partly) based on the disk's serial number, and ZFS will report the disk as missing entirely (often simply reporting it by GUID).

Sidebar: Which disk name ZFS uses when bringing up a pool

Which name or form of device identification ZFS uses is a bit complicated. To simplify a complicated situation (see vdev_disk_open in vdev_disk.c) as best I can, the normal sequence is that ZFS starts out by trying the filesystem path but verifying the devid. If this fails, it tries the devid, the physical path, and finally the filesystem path again (but without verifying the devid this time).

Since ZFS verifies the disk label's GUID and other details after opening the disk, there is no risk that finding a random disk this way (for example by the physical path) will confuse ZFS. It'll just cause ZFS to report things like 'missing or corrupt disk label' instead of 'missing device'.

Written on 16 August 2017.
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Last modified: Wed Aug 16 23:47:46 2017
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