Re: [gentoo-user] Synchronous writes over the network.

[Rich Freeman]

On Thu, Dec 23, 2021 at 11:56 AM Mark Knecht <markknecht@×××××.com> wrote:
>
>    Thanks for the responses. I'll post a single response here. I had
> thought of the need to mirror the ZIL but didn't have enough physical
> disk slots in the backup machine for the 2nd SSD. I do think this is a
> critical point if I was to use the ZIL at all.

Yeah, I wouldn't run ZIL non-mirrored, especially if your underlying
storage is mirrored.  The whole point of sync is to sacrifice
performance for reliability, and if all it does is force the write to
the one device in the array that isn't mirrored that isn't helping.
Plus if you're doing a lot of syncs then that ZIL could have a lot of
data on it.  Plus it is an SSD that you're forcing a lot of writes
through, so that is going to increase your risk of failure at some
point.

Nobody advocates for non-mirrored ZIL, at least if your array itself
is mirrored.

> Instead
> of a ZIL in machine 1 the SSD becomes a ZLOG cache most likely holding
> a cached copy of the currently active astrophotography projects.

I think you're talking about L2ARC.  I don't think "ZLOG" is a thing,
and a log device in ZFS is just another name for ZIL (since that's
what it is - a high performance data journal).

L2ARC drives don't need to be mirrored and their failure is harmless.
They generally only improve things, but of course they do nothing to
improve write performance - just read performance.

>    As always I'm interested in your comments about what works or
> doesn't work about this sort of setup.

Ultimately it all comes down to your requirements and how you use
stuff.  What is the impact to you if you lose this real-time audio
recording?  If you will just have to record something over again but
that isn't a big deal, then what you're doing sounds fine to me.  If
you are recording stuff that is mission-critical and can't be repeated
and you're going to lose a lot of money or reputation if you lose a
recording, then I'd have that recording machine be pretty reliable
which means redundant everything (server grade hardware with fault
tolerance and RAID/etc, or split the recording onto two redundant sets
of cheap consumer hardware).

I do something similar - all the storage I care about is on
Linux/ZFS/lizardfs with redundancy and backup.  I do process
photos/video on a windows box on an NVMe, but that is almost never the
only copy of my data.  I might offload media to the windows box from
my camera, but if I lose that then I still have the camera.  I might
do some processing on windows like generating thumbnails/etc on NVMe
before I move it to network storage.  In the end though it goes to zfs
on linux and gets backed up and so on.  If I need to process some
videos I might copy data back to a windows NVMe for more performance
if I don't want to directly spool stuff off the network, but my risks
are pretty minimal if that goes down at any point.  And this is just
personal stuff - I care about it and don't want to lose it, but it
isn't going to damage my career if I lose it.  If I were dealing with
data professionally it still wouldn't be a bad arrangement but I might
invest in a few things differently.

Just ask yourself what hardware needs to fail for you to lose
something you care about at any moment of time.  If you can tolerate
the loss of just about any individual piece of hardware that's a
pretty good first step for just about anything, and is really all you
need for most consumer stuff.  Backups are fine as long as they're
recent enough and you don't mind redoing work.

-- 
Rich