Re: ZFS on Guix, again

[raid5atemyhomework]

Hi Ludo'

> Hi,
>
> Sorry for the delay; this isn’t as simple as it looks!
>
> I agree with 宋文武 regarding ‘file-system-service-type’.
>
> raid5atemyhomework raid5atemyhomework@protonmail.com skribis:
>
> > However, for the case where the user expects the "typical" ZFS style of 
> > managing file systems, we need to mount all the ZFS file systems and ensure 
> > that they aer all already mounted by the time `file-systems` Shepherd 
> > service is started. This means we need to be able to extend the 
> > `requirement` of the `file-systems` Shepherd service. And we need to do 
> > that without putting any extra `/etc/fstab` entries since for "typical" ZFS 
> > style of managing file systems, they are required to not be put in 
> > `/etc/fstab`.
>
> Looks like this fstab issue is the main reason why you felt the need to
> define an extra service type. Why is it important that ZFS not be
> listed in /etc/fstab?


Because on all non-Guix operating systems, they aren't listed in `/etc/fstab`:

* https://docs.oracle.com/cd/E19120-01/open.solaris/817-2271/gaztn/index.html

What ZFS users expect is that you just do something as simple as this:

    # zpool create mypool raidz2 /dev/disk/by-id/ata-Generic_M0D3L_53R14LN0 
/dev/disk/by-id/ata-Generic_M0D3L_53R14LN1  
/dev/disk/by-id/ata-Generic_M0D3L_53R14LN2  
/dev/disk/by-id/ata-Generic_M0D3L_53R14LN3 log mirror 
/dev/disk/by-id/ata-Generic_55DM0D3L_53R14LN0  
/dev/disk/by-id/ata-Generic_55DM0D3L_53R14LN1 
/dev/disk/by-id/ata-Generic_55DM0D3L_53R14LN2

And what happens is:

* The pool `mypool` is created containing a RAIDZ-2 of the 4 HDDs listed, with 
a separate log device consisting of a mirror of 3 SSDs.
* A filesystem `mypool` is created on the pool `mypool`.
* The `mypool` filesystem is mounted on `/mypool`.
* On all subsequent bootups, the `mypool` filesystem is mounted on `/mypool`.

In ZFS you are expected to have dozens of filesystems.  If you have a new 
application, the general expectation is that you create a new filesystem for 
it.  In general you might have one pool, or maybe two or three, but you host 
most of your data in multiple filesystems on that same pool.

So for example you might want to create a filesystem for videos, which are 
sequentially accessed and tend to be fairly large, so setting `recordsize=1M` 
makes sense (good for sequential access, not so much for random, and good for 
very large files measurable in dozens of megabytes).

    # zfs create -o recordsize=1M -o mountpoint=/home/raid5atemyhomework/Videos 
mypool/videos

The above command does:

* The filesystem `videos` is created on the pool `mypool`.
* The `mypool/videos` filesystem is mounted on 
`/home/raid5atemyhomework/Videos`.
* On all subsequent bootups, the filesystem is mounted on 
`/home/raid5atemyhomework/Videos`.

Now I might also want to run say a PostgreSQL service.

* PostgreSQL allocates in page sizes of 8k, so `recordsize=8k` is best.
* PostgreSQL uses a journal, which has a different access pattern from the rest 
of the data.  Journals are written sequentially and read sequentially, while 
the database itself is accessed randomly.
  * The data should have `logbias=throughput` to optimize and reduce use of the 
ZIL SLOG, to avoid "log on a log" slowdown effects.
  * The journal itself should continue to use the default "latency".

So I would do:

    # zfs create -o recordsize=8k -o logbias=throughput -o 
mountpoint=/postgresql mypool/postgresql
    # zfs create -o logbias=latency -o mountpoint=/postgresql/pg_wal 
mypool/postgresql/pg_wal

That means creating two filesystems for a single application, one for the 
PostgreSQL data, the other for the PostgreSQL journal.

What the above examples show is:

* The habit for a ZFS user is to create many filesystems.  On my own homelab I 
have two filesystems (one for documents and code, one for videos and pictures) 
for data I manage myself, and I have two other filesystems for two different 
applications I am running as well.
* Each filesystem has different tuning properties.

On a server you might have a dozen or so ZFS filesystems for various 
applications you need to run.  There are also many other tuning parameters to 
tweak.  If done by `/etc/fstab` it would lead to a fairly large file.

The base logic here is that `/etc/fstab` has to be stored on disk anyway, and 
ZFS can just store the same information on the disks it is managing directly.  
Then ZFS supports nice tabulated output of properties via `zfs list`:

    # zfs list -o name,recordsize,logbias,atime,relatime
    NAME                RECSIZE  LOGBIAS     ATIME  RELATIME
    hddpool                128K  latency     off    on
    hddpool/bitcoin        128K  latency     off    on
    hddpool/common         128K  latency     off    on
    hddpool/lightning       64K  latency     off    on
    hddpool/media            1M  latency     off    on

And you can change parameters easily with `zfs set`.  There are many dozens of 
possible properties as well.

Thus, the general expectation among ZFS users is ***not*** to use any kind of 
`/etc/fstab` at all, because such a `/etc/fstab` would be ludicrously large 
with a dozen filesystems and several properties.  And the declarative 
`file-system` Guix syntax is really just `/etc/fstab` in another format.  So 
the expectation for a ZFS user would be to keep using classic `zpool` and `zfs` 
commands to manage the filesystems and parameters.

The main purpose of the `operating-system` declaration is to allow the system 
to be brought back again, but the configuration file has to exist on *some* 
permanent storage anyway so the information might as well be managed by ZFS 
directly on the disks it is managing.

ZFS also allows snapshotting of the configuration of the pool, so this isn't 
really an advantage to keeping the configuration of the pool in the 
`operating-system` as well.  You can rollback changes to the pool just as well 
as you can rollback `operating-system`.

Since this is the expected behavior of ZFS, we should support it as much as 
possible.

If the user wants to really manage ZFS via `file-system` declarations, they can 
set `mountpoint=legacy` and then the user can put them in `file-system` 
declarations that then become `/etc/fstab` entries.  But if the user doesn't 
want to manage them via `file-system` declarations, we should also support that 
use-case (because that is how ZFS is meant to be used in other operating 
systems).  So we need to have the `file-system` Shepherd service also wait for 
non-`/etc/fstab` filesystems like ZFS, not just those listed in `/etc/fstab`.


Thanks
raid5atemyhomework