[Gluster-users] 3.7.16 with sharding corrupts VMDK files when adding and removing bricks

[Gandalf Corvotempesta]

1016-11-14 17:01 GMT+01:00 Vijay Bellur <vbellur at redhat.com>:
> Accessing sharded data after disabling sharding is something that we
> did not visualize as a valid use case at any point in time. Also, you
> could access the contents by enabling sharding again. Given these
> factors I think this particular problem has not been prioritized by
> us.

That's not true.
If you have VMs running on a sharded volume and you disable sharding,
with the VM still running, everything crash and could lead to data loss, as VM
will be unable to find their filesystem and so on, qemu currupts the
image and so on.....

If I write to a file that was shareded, (in example a log file), now
when you disable the shard,
the application would write the existing file (the one that was the
first shard).
If you reenable sharding, you lost some data

Example:

128MB file. shard set to 64MB. You have 2 chunks: shard1+shard2

Now you are writing to the file:

AAAA
BBBB
CCCC
DDDD

AAAA+BBBB are placed on shard1, CCCC+DDDD are placed on shard2

If you disable the shard and write some extra data, EEEE, then EEEE
would be placed after BBBB in shard1 (growing more than 64MB)
and not on shard3

If you re-enable shard, EEEE is lost, as gluster would expect it as
shard3. and I think gluster will read only the first 64MB from shard1.
If gluster read the whole file, you'll get something like this:

AAAA
BBBB
EEEE
CCCC
DDDD

in a text file this is bad, in a VM image, this mean data
loss/corruption almost impossible to fix.


> As with many other projects, we are in a stage today where the number
> of users and testers far outweigh the number of developers
> contributing code. With this state it becomes hard to prioritize
> problems from a long todo list for developers.  If valuable community
> members like you feel strongly about a bug or feature that need
> attention of developers, please call such issues out on the mailing
> list. We will be more than happy to help.

That's why i've asked for less feature and more stability.
If you have to prioritize, please choose all bugs that could lead to
data corruption or similiar.