[linux-lvm] bug? shrink lv by specifying pv extent to be removed does not behave as expected

[Zdenek Kabelac]

Dne 09. 04. 23 v 20:21 Roland napsal(a):
>> Well, if the LV is being used for anything real, then I don't know of
>> anything where you could remove a block in the middle and still have a
>> working fs.   You can only reduce fs'es (the ones that you can reduce)
> 
> my plan is to scan a disk for usable sectors and map the logical volume
> around the broken sectors.
> 
> whenever more sectors get broken, i'd like to remove the broken ones to have
> a usable lv without broken sectors.
>

Reall silly plan  - been there years back in time when drives were FAR more 
expensive with the price per GiB.

Todays - just throw bad drive to recycle bin - it's not worth to do this 
silliness.

HDD bad sectors are spreading - and slowly the surface gets destroyed....

So if you make large 'head-room' around bad disk areas - if they are 
concentrated on some disk area - and you know topology of you disk drive
like i.e. 1% free disk space before and after bad area - you could possibly 
use disk for a little while more - but only to store invaluable data....


> since you need to rebuild your data anyway for that disk, you can also
> recreate the whole logical volume.
> 
> my question and my project is a little bit academic. i'd simply want to try
> out how much use you can have from some dead disks which are trash otherwise...

You could always take  'vgcfgbackup'  of lvm2 metadata and make some crazy 
transition of if with even  AWK/python/perl   -  but we really tend to support
just some useful features - as there is already  'too much' and users are 
getting often lost.

One very simply & naive implementation could be going alonge this path -

whenever you want to create new arrangement for you disk with 'bad' areas,
you can always start from 'scratch' - since afterall - lvm2 ONLY manipulates 
with metadata within disk front - so if you need to create new 'holes',
just   'pvcreate -f', vgcreate,   and 'lvcreate -Zn -Wn'
and then  'lvextend'  with normal  or  'lvextend --type error | --type zero' 
segment types around bad areas with specific size.
Once you are finished and your LV precisely matches your 'previous'  LV of you 
past VG - you can start to use this LV again with new arrangement of  'broken 
zeroed/errored' areas.

I've some serious doubts about usability of this with any filesystem :) but if 
you think this has some added value - fell free to use.
If the drive you play with would be 'discardable' (SSD/NVMe) then one must 
take extra care there is no 'discard/TRIM' anywhere in the process - as that 
would lose all data irrecoverably....

But good advice from me - whenever  'smartctl' starts to show relocation block 
errors - it's the right moment to  'dd_rescue' any LV to your new drive...
> 
> yes, pvmove is the other approach for that.
> 
> but will pvmove continue/finish by all means when moving extents located on a
> bad sector ?

pvmove  CANNOT be used with bad drives - it cannot deal with erroring sectors 
and basically gets stuck there trying to mirror unrecoverable disk areas...

Regards

Zdenek