Hi David,
Sorry for the delay. Yes, at this point I feel it would be best to
continue this discussion off-list, or perhaps to shift it to the
debian-doc list. Appologies to linux-btrfs if this should have been
shifted sooner! I'll follow-up with a PM reply momentarily.
Cheers,
Nicholas
On 3 May 2016 at 03:37, David Alcorn <nrocl...@gmail.com> wrote:
> "Honestly, did you read the Debian wiki pages for btrfs and EFI? If
> you read them, could you please let me know where they were deficient
> so I can fix them?"
>
> I did not use the Debian wiki pages for BTRFS and UEFI as a resource
> in my attempts to answer my questions because I read them in the past
> and they did not address my specific needs. Technically, I lack the
> skill set required for my perspectives to merit credulity but I am
> willing to give it a shot. I do not want to take the list off focus:
> if this discussion belongs elsewhere, let me know.
>
> My question about how to recover/replace a failed boot where "/" is
> located in a BTRFS subvolume located on a BTRFS RAID56 array presents
> challenges but it is reasonable to provide sufficient infrastructure
> in the wiki's to let a portion of the readers answer this question
> themselves rather than bother this list. Am I correct that (i) there
> is no reasonable tool to permit a screen shot of the Grub menu being
> edited using the "e" key as the O/S has not yet loaded?, and (ii) do
> USB flash drive (unlike some SSD's) respect the "dup" data profile?
>
> It is easy to answer my question whether "/boot" may be located on a
> BTRFS RAID56 array somewhere in the UEFI wiki. I am more comfortable
> with a more comprehensive revision to the wiki as suggested in the
> below draft. If the editorial comments are excessive or offend
> community standards, scrap em.
>
> Replace the "RAID for the EFI System Partition" section with:
>
> "DRAFT: RAID and LVM for the EFI and /Boot Partitions". The UEFI
> firmware specification supports several alternative boot strategies
> including PXE boot and boot from an EFI System Partition ("ESP") which
> might be located on a MBR, GPT or El Torito volume on an optical disk.
> The ESP must be partitioned using a supported FAT partition (such as
> FAT32). A mdadm RAID array (other than perhaps a RAID 1 array
> formatted as FAT32), a LVM partition and a BTRFS RAID array are not
> FAT and can not hold a functional ESP. Once Grub loads the ESP
> payload, Grub has enhanced abilities to recognize file systems which
> it uses to acquire required information from "/boot". The Grub
> Manual, which may be viewed with the command "info grub", reports Grub
> (unlike grub-legacy stage 1.5) has some ability to use advanced file
> systems such as LVM and RAID once the ESP payload is loaded. This
> support appears to exclude BTRFS RAID 56. Other than the possible
> mdadm RAID 1 exception noted above, ESP always goes in a separate, non
> array, non LVM FAT partition. For BTRFS RAID56 arrays, "/boot" also
> requires a separate, non array partition.
>
> Because LVM does not favor a whole disk Physical Volume ("PV") over a
> partition based PV, it is trivial to create a petite ESP on a disk and
> assign the balance of the disk to a LVM PV. Array capacity of both
> MDADM and BTRFS RAID 56 arrays may be disproportionately reduced when
> the size of a single disk is reduced by, say an ESP. For
> administrative simplicity and to maximize array capacity, equal sized
> whole disk arrays are favored.
>
> Both the ESP and "/boot" partitions present limited, read dominated
> workloads. USB flash drives are cheap and tolerate light, read
> dominated workloads well. For a stand alone server, it is common to
> locate the ESP on a USB flash device. If you use a BTRFS RAID56
> array, "/boot" and perhaps "/swap" may also go to separate partitions
> on the flash drive. This permits assignment of whole disks to the
> array. If you are working with a large number of servers, it may be
> cheaper, more energy efficient, and more reliable to replace whatever
> is on the flash drive with PXE boot. Frequently, SATA (or IDE) drives
> that are not wholly allocated to the RAID array are scarce. If you
> have one, the ESP (and "/boot") partitions may be located there.
> Similar concerns affect LILO.
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