[linux-lvm] Lvm think provisioning query

Zdenek Kabelac zkabelac at redhat.com
Tue May 3 09:54:56 UTC 2016

On 3.5.2016 08:59, Bhasker C V wrote:
> Does this mean the ext4 is showing wrong information. The file is reported
> being 90+MB but in actuality the size is less in the FS ?
> This is quite ok because it is just that file system being affected. I was
> however concerned that the file in this FS might have overwritten other LV
> data since the file is showing bigger than the volume size.

I've no idea what 'ext4' is showing you, but if you have i.e. 100M filesystem 
size, you could still have there e.g. 1TB file. Experience the magic:

'truncate -s 1T myfirst1TBfile'

As you can see 'ext4' is doing it's own over-provisioning with 'hole' files.
The only important bits are:
- is the filesystem consistent ?
- is 'fsck' not reporting any error ?

What's the 'real' size you get with 'du  myfirst1TBfile' or your wrong file ?

Somehow I don't believe you can get  i.e.  90+MB 'du' size with 10MB 
filesystem size and 'fsck' would not report any problem.

> I will try this using BTRFS.

For what exactly ??



> On Fri, Apr 29, 2016 at 10:13 AM, Zdenek Kabelac <zkabelac at redhat.com
> <mailto:zkabelac at redhat.com>> wrote:
>     On 28.4.2016 16:36, Bhasker C V wrote:
>         Zdenek,
>            Thanks. Here I am just filling it up with random data and so I am not
>         concerned about data integrity
>            You are right, I did get page lost during write errors in the kernel
>         The question however is even after reboot and doing several fsck of
>         the ext4fs
>         the file size "occupied" is more than the pool size. How is this ?
>         I agree that data may be corrupted, but there *is* some data and this
>         must be
>         saved somewhere. Why is this "somewhere" exceeding the pool size ?
>     Hi
>     Few key principles -
>     1. You should always mount extX fs with  errors=remount-ro  (tune2fs,mount)
>     2. There are few data={} modes ensuring various degree of data integrity,
>         An case you really care about data integrity here - switch to 'journal'
>         mode at price of lower speed. Default ordered mode might show this.
>         (i.e. it's the very same behavior as you would have seen with failing hdd)
>     3. Do not continue using thin-pool when it's full :)
>     4. We do miss more configurable policies with thin-pools.
>         i.e. do plan to instantiate 'error' target for writes in the case
>         pool gets full - so ALL writes will be errored - as of now - writes
>         to provisioned blocks may cause further filesystem confusion - that's
>         why  'remount-ro' is rather mandatory - xfs is recently being enhanced
>         to provide similar logic.
>     Regards
>     Zdenek
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