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dm thin: relax hard limit on the maximum size of a metadata device

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The thin metadata format can only make use of a device that is <=
THIN_METADATA_MAX_SECTORS (currently 15.9375 GB).  Therefore, there is no
practical benefit to using a larger device.

However, it may be that other factors impose a certain granularity for
the space that is allocated to a device (E.g. lvm2 can impose a coarse
granularity through the use of large, >= 1 GB, physical extents).

Rather than reject a larger metadata device, during thin-pool device
construction, switch to allowing it but issue a warning if a device
larger than THIN_METADATA_MAX_SECTORS_WARNING (16 GB) is
provided.  Any space over 15.9375 GB will not be used.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
This commit is contained in:
Mike Snitzer
2012-03-28 18:41:28 +01:00
committed by Alasdair G Kergon
parent 71fd5ae25d
commit c4a69ecdb4
4 changed files with 25 additions and 18 deletions
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8
Documentation/device-mapper/thin-provisioning.txt
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@@ -75,10 +75,12 @@ less sharing than average you'll need a larger-than-average metadata device.
As a guide, we suggest you calculate the number of bytes to use in the
metadata device as 48 * $data_dev_size / $data_block_size but round it up
to 2MB if the answer is smaller. The largest size supported is 16GB.
to 2MB if the answer is smaller. If you're creating large numbers of
snapshots which are recording large amounts of change, you may find you
need to increase this.
If you're creating large numbers of snapshots which are recording large
amounts of change, you may need find you need to increase this.
The largest size supported is 16GB: If the device is larger,
a warning will be issued and the excess space will not be used.
Reloading a pool table
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