* Re: mirroring existing boot drive sanity check
2022-08-09 14:50 mirroring existing boot drive sanity check David T-G
@ 2022-08-10 7:03 ` Wols Lists
2022-08-11 21:26 ` Pascal Hambourg
1 sibling, 0 replies; 5+ messages in thread
From: Wols Lists @ 2022-08-10 7:03 UTC (permalink / raw)
To: David T-G, linux-raid
On 09/08/2022 15:50, David T-G wrote:
> Hi, all --
>
> I think that this has been asked before, and so I think that I know
> where I'm
> going, but before I take aim at my foot with a large-caliber mdadm ... :-)
>
> I have an existing
>
> diskfarm:~ # parted /dev/sda unit MiB print free
> Model: ATA SanDisk SD6SB1M1 (scsi)
> Disk /dev/sda: 122104MiB
> Sector size (logical/physical): 512B/512B
> Partition Table: gpt
> Disk Flags: pmbr_boot
>
> Number Start End Size File system Name Flags
> 0.02MiB 1.00MiB 0.98MiB Free Space
> 1 1.00MiB 33793MiB 33792MiB linux-swap(v1) diskfarm-swap
> swap
> 2 33793MiB 66561MiB 32768MiB xfs diskfarmsuse
> 3 66561MiB 99329MiB 32768MiB diskfarmknop
> legacy_boot
> 4 99329MiB 122104MiB 22775MiB xfs diskfarm-ssd
>
> 128G SSD. I have obtained a shiny new
>
> diskfarm:~ # parted /dev/sde unit MiB print free
> Model: ATA SATA SSD (scsi)
> Disk /dev/sde: 244198MiB
> Sector size (logical/physical): 512B/512B
> Partition Table: gpt
> Disk Flags:
>
> Number Start End Size File system Name Flags
> 0.02MiB 1.00MiB 0.98MiB Free Space
> 1 1.00MiB 33793MiB 33792MiB diskfarm-swap swap
> 2 33793MiB 66561MiB 32768MiB diskfarmsuse
> 3 66561MiB 99329MiB 32768MiB diskfarmknop
> legacy_boot
> 4 99329MiB 122104MiB 22775MiB diskfarm-ssd
> 122104MiB 244198MiB 122094MiB Free Space
>
> 256G SSD to use as a mirror. [You can ignore the sgdisk-copied partition
> layout for the moment; that was a false start.] My final-view plan is, in
> fact, to replace the 128 with another 256 and grow the -ssd data partition.
>
> For a typical mirror-an-existing, I think that I need to create all of my
> slices and the [degraded] mirror on the new, copy over the old, boot
> from new,
> and then treat old as just another disk to shove in. There's the
> question of
> making partitions larger for the RAID superblock info, though, and -- and
> here's where I get confused -- even on the old disk when adding it in.
https://raid.wiki.kernel.org/index.php/Converting_an_existing_system
>
> As you can see, I have no free space on the little guy. I was thinking I'd
> bump my slices larger on the new disk so that I have room to spare to copy
> everything over, with the data slice a little less larger than it would
> have
> been, but then ... I think I saw that I need to make the 2nd-drive slices
> larger, too, so what do I do with the old guy?
Yup. If your old system is full (or even if it isn't), if you're moving
a straight partition on to raid, it's easier just to create all your
slices new on the new system and move across.
If you really want to re-use the old drive, then you've got some maths
ahead of you ...
You need to allocate the planned partition to your raid.
Then you create the raid, telling it to only use a space equal or less
than your original disk.
Then you copy your original filesystem into the raid, except it probably
doesn't quite fit, so you have to shrink it (not a simple matter) or use
cp or rsync instead, equally unpleasant.
Then you wipe your old drive and add it as the new raid member.
It's probably not hard. But it's got vastly more scope for error,
cock-up, fat-finger, or plain hardware hiccup. Is it worth it ...
>
> And, in fact, does it even matter? If I understand this correctly, I'll be
> running entirely from the new disk in the mirror once this is done, and
> so it
> doesn't matter whether I put the little old or the other big new in to
> fill out
> the mirror. If that's the case, then I don't really care about
> partition size
> because I'm going to start with mirrored partitions.
Yup again. You're better off just putting in a new 256GB.
Then you can just dd your old root (and whatever else) filesystem(s)
across, and grow them into the new space.
Plus your old drive is now just sitting there as a backup.
>
> Oh, and just because I'm a glutton for punishment (even more than using
> this
> stupid webmail because we're currently down my home directory disk on
> our mail
> server and I'm impatient), if I'm essentially starting from scratch,
> should I
> mirror the entire [yes, identical] drive and partition the metadevice,
> *BSD-style, or mirror individual partitions?
>
As my wife says about my driving, she trusts me, but there are too many
idiots out there. Advice is *ALWAYS* partition your hard drive. raid
couldn't care, but there are too many idiots out there that assume an
unpartitioned drive is empty, and will stomp on it without asking. It's
fallen off, but probably the biggest single cause of raid recoveries
here is "something overwrote my superblock with a partition table" or
the like - it's usually partition-related ...
Cheers,
Wol
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: mirroring existing boot drive sanity check
2022-08-09 14:50 mirroring existing boot drive sanity check David T-G
2022-08-10 7:03 ` Wols Lists
@ 2022-08-11 21:26 ` Pascal Hambourg
2022-08-12 7:23 ` Wols Lists
1 sibling, 1 reply; 5+ messages in thread
From: Pascal Hambourg @ 2022-08-11 21:26 UTC (permalink / raw)
To: David T-G, linux-raid
Le 09/08/2022 à 16:50, David T-G a écrit :
>
> I have an existing 128G SSD.
>
> Disk /dev/sda: 122104MiB
> Sector size (logical/physical): 512B/512B
> Partition Table: gpt
> Disk Flags: pmbr_boot
>
> Number Start End Size File system Name Flags
> 0.02MiB 1.00MiB 0.98MiB Free Space
> 1 1.00MiB 33793MiB 33792MiB linux-swap(v1) diskfarm-swap swap
> 2 33793MiB 66561MiB 32768MiB xfs diskfarmsuse
> 3 66561MiB 99329MiB 32768MiB diskfarmknop legacy_boot
> 4 99329MiB 122104MiB 22775MiB xfs diskfarm-ssd
>
> I have obtained a shiny new 256G SSD to use as a mirror.
> My final-view plan is, in
> fact, to replace the 128 with another 256 and grow the -ssd data partition.
>
> For a typical mirror-an-existing, I think that I need to create all of my
> slices and the [degraded] mirror on the new, copy over the old, boot from new,
> and then treat old as just another disk to shove in. There's the question of
> making partitions larger for the RAID superblock info
If you choose to copy existing block device content (with dd or the
like) into a RAID array, then the RAID array device size must be at
least the same this, which implies that the RAID member devices is
sligthly bigger to take the RAID superblock into account.
If you choose to copy filesystem content (with cp, rsync or the like)
into a new filesystem, then you only need the RAID array device to be
big enough to fit the content.
> As you can see, I have no free space on the little guy.
Actually no, we cannot see. We can only see that there is no free space
outside the partitions. But we cannot see if there is any free space
inside the partitions.
> what do I do with the old guy?
Do whatever you like with the old drive except using it in the RAID
array. Why bother doing this and having to resize the RAID array when
you add the 2nd new drive ? Resizing a RAID array is a pain in the ass.
Just build the RAID array on the 2 new drives from the start.
> if I'm essentially starting from scratch, should I
> mirror the entire [yes, identical] drive and partition the metadevice,
> *BSD-style, or mirror individual partitions?
IMO a single RAID array is simpler. If your distribution supports it,
you can either partition it with a partition table or use it as a LVM
physical volume and create logical volumes.
However I do not think it is possible to cleanly boot from an
unpartitioned drive used as a software RAID member, as a RAID capable
boot loader could hardly fit in the 4-KiB area before the RAID
superblock. So you still have to create a partition table on the raw
drives. Also, if you use GPT format and GRUB boot loader, you need to
create a small (100 kB to 1 MB) partition with type "BIOS boot" (or
libparted bios_grub flag).
^ permalink raw reply [flat|nested] 5+ messages in thread