Bug 1314770 - lvcreate can't always create thin pool of maximum size
lvcreate can't always create thin pool of maximum size
Product: Fedora
Classification: Fedora
Component: lvm2 (Show other bugs)
Unspecified Unspecified
unspecified Severity unspecified
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Assigned To: Zdenek Kabelac
Fedora Extras Quality Assurance
Depends On:
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Reported: 2016-03-04 07:39 EST by Marius Vollmer
Modified: 2016-12-20 14:14 EST (History)
12 users (show)

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Doc Type: Bug Fix
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Last Closed: 2016-12-20 14:14:27 EST
Type: Bug
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Description Marius Vollmer 2016-03-04 07:39:32 EST
Description of problem:

The lvcreate command fails when trying to create a thin pool of maximum size by giving the exact size (instead of 100%FREE).  After failure, an unexpected volume group of minimal size has been created.

Version-Release number of selected component (if applicable):

How reproducible:

Steps to Reproduce:

# vgs -o name,vg_extent_size,vg_free_count --units b
  VG   Ext      Free
  vg   4194304B  510

# lvcreate vg -T --thinpool pool -l 510 
  Volume group "vg" has insufficient free space (509 extents): 510 required.

# lvs -o name,lv_size
  LV    LSize
  lvol0 4.00m

# lvremove vg/lvol0
  Logical volume "lvol0" successfully removed

# lvcreate vg -T --thinpool pool -l 509
  Insufficient free space: 254 extents needed, but only 253 available

# lvremove vg/lvol0
  Logical volume "lvol0" successfully removed

# lvcreate vg -T --thinpool pool -l 508
  Logical volume "pool" created.

# lvs -o name,lv_size,lv_metadata_size --units=4194304B
  LV   LSize   MSize 
  pool 508.00U  1.00U

(One extent has disappeared, no?)

Actual results:

As above.

Expected results:

lvcreate succeeds to create a thin pool when given the number of free extents in the volumegroup.

When lvcreate fails, it should not leave an unexpected logical volume behind.

Additional info:

The actual volume group in question is very small, only 2 GiB.  I try with a larger one also.

Using "-L" fails in the same way.

Using "-l 100%FREE" succeeds, but we are using lvcreate as an API, so it would be nice to just pass raw numbers.
Comment 1 Marius Vollmer 2016-03-04 07:40:22 EST
> After failure, an unexpected volume group of minimal size has been created.

Sorry, an unexpected logical volume is created.
Comment 2 Marius Vollmer 2016-03-04 07:57:04 EST
> I try with a larger one also.

Similar results with a 30 GiB volume group:

# vgs -o name,vg_extent_size,vg_free_count --units b VG
  VG   Ext      Free
  VG   4194304B 7679
# lvcreate VG -T --thinpool pool -l 7679
  Volume group "VG" has insufficient free space (7671 extents): 7679 required.

Unfortunately, lcreate doesn't always miscalculate by exactly 2 extents as in comment 0.  What would a good way be to calculate the maximum number that can be given to "-l"?
Comment 3 Zdenek Kabelac 2016-03-04 08:08:05 EST
Not sure what you trying to achieve - but  thin-pool is being build from 2 underlying LVs -  one is  dataLV and the other is metadataLV.

The size of metadataLV is estimated by lvm2 tool based on our 'user-experience' estimation and changes - it's not fixed but at the moment we roughly target to be at most 128MB unless told differently.

Then there is hidden sparse 3rd. LV  (see lvs -a   _pmspare)  which is our placeholder within VG to be able to repair thin-pool metadata.
(so yep 3 LVs are being made - and size of those extra 2 are not fixed and may change according to our needs).

So there is nothing like use  100% just for my data.

In fact you should not be even trying to do it this way - you should start with reasonable 'minimal' size of thin-pool and let the thin-pool automatically grow once its space is exhausted/used - so the remaining portion of VG could be used dynamically used for something else.

If there is ANY tool expecting some fixed sizes - the tool is doing it wrong.
Sizes are under full control of lvm2 - unless explicitly specified on commands line - but in that case you loose crucial feature of lvm2 to manage your volumes in the best way...
Comment 4 Marius Vollmer 2016-03-04 09:05:58 EST
Okay, I think I get it now.  The size of the meta data and potential sparse volume are added to the value given to -l  to give the total size, instead of being subtracted to give the data volume size, except when using the %FREE convenience syntax.

So it looks like we need to use the %FREE syntax as well as the API when we want to say "take this amount of space and turn it into a thin pool with all your defaults" instead of "make me a thin pool for this much data and use as much extra space as is needed according to your defaults".
Comment 5 Marius Vollmer 2016-03-04 09:10:20 EST
> Not sure what you trying to achieve

We try to make a super simple UI for thin pool creation where the user inputs a name and indicates the size with a slider.  Whacking the slider to the right should result in a maximally big thin pool.
Comment 6 Marius Vollmer 2016-03-04 09:11:48 EST
Please note that while it might be OK for lvcreate to fail in this case, it should not leave the expected logical volume behind.
Comment 7 Zdenek Kabelac 2016-03-04 09:24:12 EST
The current size logic does apply to some 'final' volume size.

So when you use  -l #extents -  there is going to be some LV which does have 
this amount of extents (or more depending on strip rounding or so).

lvm2 currently doesn't have an option like --totalextents -  where you would let lvm2 figure out internal subvolumes to fit totally into given number of extents.

With -l%FREE the situation is slight different - here is lvm2 given some 'freedom' and it will try to adapt size to build something as result - and when it  fails - it should be mostly seen as a bug (and I think there are quiet a room for improvements - but ATM it's complicated to fix)

Also it's really time to move from 'extent' precision - users do not care if the volume is 1 extent bigger or smaller - when you operate in TB.
Comment 8 Marius Vollmer 2016-03-04 09:37:07 EST
(In reply to Zdenek Kabelac from comment #7)

> lvm2 currently doesn't have an option like --totalextents -  where you would
> let lvm2 figure out internal subvolumes to fit totally into given number of
> extents.

This would be very useful for us.
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