Making gmirror metadata cooperate with gpt metadata

[Paul Mather]

On Feb 7, 2021, at 4:23 PM, Abner Gershon <6731955 at gmail.com> wrote:

> Wow, thanks for opening my eyes to this. I did not realize that BSD
> partitions may be layered on top of a GPT partition.


Hopefully it was clear from my original reply that I wasn't sure you could do this and you should try it out for yourself (e.g., in a VM or using an md-disk). :-)

It's not clear whether it is possible from the gpart man page.  For the BSD partitioning scheme it says, "Traditional BSD disklabel, usually used to subdivide MBR partitions.  (This scheme can also be used as the sole partitioning method, without an MBR. ..."  It's not clear to me whether you could create a partitioning scheme in this way and still have the resultant system boot via EFI or legacy BIOS---it's the latter "being able to boot" which is the most important, IMHO.


> If I understand this correctly, you are suggesting I use fdisk to partition
> a GPT partition?


No, my thought was just to add a partition of type "freebsd" inside the GPT.  I do note that the gpart man page discourages its use: "This is a legacy partition type and should not be used for the APM or GPT schemes."  Then, as I said above, there is the matter of whether a FreeBSD boot loader could successfully boot from such a layout. :-\


> My concern with gmirror on partition level is that I have seen a comment
> about how this may cause excessive stress on disk due to contention between
> various sectors for writing data during initial mirroring. In other words
> will gmirror update one partition at a time or will disk write head jitter
> back and forth writing 4k to 1st partition, 4k to 2nd partition, 4k to 3rd
> partition, etc.


To be honest, I don't remember what it does because I only use gmirror for swap nowadays, but I have a sneaking suspicion from memory that it was subject to the jitter you mention (at least years ago when I was still gmirroring UFS filesystems).  I ended up turning off autosynchronisation and doing it myself on the occasions when the mirror broke.

For initial mirroring you could make a special case for synchronising, if you were worried about disk stress.  People are increasingly using SSDs for OS drives nowadays, so stress from mechanical head movement becomes a moot point in that case.  (In fact, all those layout and rotational optimisations in UFS designed to minimise physical head movement and rotational latency become moot in that case.)


> I have been resisting ZFS because of inefficiencies of COW for updating
> database files ( as opposed to updating one block of existing file ).


Don't databases themselves use COW techniques to ensure data safety?


> I
> plan to set up a server with some UFS gmirror and some ZFS storage and do
> some comparisons. Will post back my results when I do. Most related posts I
> have seen suggest ZFS is the way of the now/future but then again I am
> driving a 1988 Ford ranger with manual transmission.


There's nothing wrong with sticking with what you know and what you feel comfortable with, and with what you believe best accommodates your use case.

Others in this thread have made some great points regarding not dismissing ZFS as an option.  I agree with what they said.  I've used FreeBSD since version 3 and used ZFS from the moment it was available in FreeBSD (version 7).  Here's what I would add/echo to what has already been said as plus points for ZFS:

- Pooled storage: no more gnashing teeth over badly sizing your filesystems
- Snapshots: cheap and reliable; I never felt the same way about UFS snapshots
- Flexible filesets: tune the behaviour of "filesytems" according to use cases
- Integrity and durability: advanced "RAID" setups and data integrity protections throughout
- Administration: better control over administration and delegation of your file systems
- Efficiency: tiered storage model

As regards ZFS being "new," it would be fair to say that it has received more active development in the last few years than UFS.  I actually switched from UFS to ZFS on FreeBSD/arm64 (on a 1 GB Raspberry Pi) because I was tired of losing data due to UFS+SUJ crashing with non-fsck-able file systems.  Snapshots on UFS have also had a chequered history of working properly (e.g., for consistent dumps).  Data safety is the #1 thing that attracts me to ZFS.

Hopefully, data safety is important to you, too.  Also, one big concern I would have in changing the semantics of gmirror, as you propose, is the easy availability of rescue tools should something happen to my storage causing everything to go pear shaped. :-)  You'd have to factor it into your disaster recovery plan.

Cheers,

Paul.