Re: [gentoo-user] LVM and moving things around

[Wol]

On 03/04/2022 19:37, Frank Steinmetzger wrote:
> Am Sun, Apr 03, 2022 at 05:05:07PM +0100 schrieb Wol:
>
>>>>> Rsync has a bwlimit argument which helps here. Note that rsync copies
>>>>> the whole file on what it considers local storage (which can be mounted
>>>>> network shares) ... this can cause a real slowdown.
>>>> It won't help on the initial copy, but look at the - I think it is -
>>>> --in-place option.
>>
>>> This one is mostly useful if space on the destination is tight or the data
>>> link (for FS commands) is slooow, because normally rsync creates a new temp
>>> file and moves it into place once the transfer is complete.
>
>> And making a temporary file may be exactly what you DON'T want. I make heavy
>> use of hard links,
>
> … which the -H option is for.
>
>> and "make a temp file" absolutely buggers file system integrity ...
>
> Can you elaborate? A temp file is just like any other file. If you do
> in-place syncing, then it “buggers up” the original file instead of a temp.

"Make a temp file" breaks hard links. My file system is full of them, 
and keeping track of hard links can easily bring a system to its knees 
(it brought a previous system of mine to its knees :-)

>
>> And my use case with LVM and backups, "make a temp file" does both
>> absolutely nothing to protect file system integrity
>
> I was not actually talking about file *system* integrity, but that if you
> overwrite in-place and the connection is lost during transfer, you are left
> with an incomplete file. Hence you just lost data on the destination. The FS
> itself is not adversely affected by that, only the file.

If the link is lost (unlikely on my system where both drives are on the 
same system) you just start again ... and you haven't actually lost any 
data, you've just got a corrupt copy to recover.
>
>> and makes every backup waste far more space than is necessary ...
>
> How so? If it’s a COW filesystem, I can understand it (insofar as if you do
> in-place, then the existing file is deleted first, which gives the FS more
> space for the new copy).
>
An LVM snapshot *IS* a COW filesystem. And no, --in-place doesn't delete 
the original, it just overwrites the bits that have changed.

So my backups, all the identical data is de-duplicated. I have two 
separate filesystems I can mount, each of which is a full backup, but 
they only take up disk space for "one and a bit".

>> So --in-place actually has a lot of uses outside your two examples. I have
>> oodles of space, and both my source and target are on fast sata links in the
>> same computer, but not using --in-place would be *very* costly for me.
>
> Well I admit I don’t know much outside of my realm of local disks with ext4,
> zfs and ssh. While I do run LVM on my laptop, it’s just so that I can have
> “partitions” on an otherwise fully encrypted block device. I never used any
> of its more fancy features.
>
Read up on it. Especially if you're running linux, snapshotting root is 
a very good idea. I always snapshot root on a friday evening then do my 
emerge, and while it's never happened, if it all goes pear-shaped I can 
mount my backup and go back to where I was. And if you're doing anything 
at all dangerous, snapshotting will protect your "pre" state.

(I only know how to make a snapshot, I've never really investigated 
rolling back, but it's nice knowing it's an option :-)

Cheers,
Wol