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From Vladimir Ozerov <voze...@gridgain.com>
Subject IGFS concurrency issue
Date Thu, 24 Sep 2015 07:32:07 GMT
Igniters,

We revealed concurrency problem in IGFS and I would like to discuss
possible solutions to it.

Consider the following file system structure:
root
|-- A
|   |-- B
|   |   |-- C
|   |-- D

... two concurrent operations in different threads:
T1: move(/A/B, /A/D);
T2: move(/A/D, /A/B/C);

... and how IGFS handles it now:
T1: verify that "/A/B" and "/A/D" exist, they are not child-parent to each
other, etc. -> OK.
T2: do the same for "A/D" and "A/B/C" -> OK.
T1: get IDs of "/A", "/A/B" and "/A/D" to lock them later inside tx.
T2: get IDs of "/A", "/A/D", "/A/B" and "/A/B/C" to lock them later inside
tx.

T1: Start pessimistic tx, lock IDs of "/A", "/A/B", "/A/D", perform move ->
OK.
root
|-- A
|   |-- D
|   |   |-- B
|   |   |   |-- C

T2: Start pessimistic tx, lock IDs of "/A", "/A/D", "/A/B" and
"/A/B/C" (*directory
structure already changed at this time!*), perform move -> OK.
root
|-- A
B
|-- D
|   |-- C
|   |   |-- B (loop!)

File system is corrupted. Folders B, C and D are not reacheable from root.

To fix this now we additionaly check if directory structure is still
valid *inside
transaction*. It works, no more corruptions. But it requres taking locks on
the whole paths *including root*. So move, delete and mkdirs opeartions *can
no longer be concurrent*.

Probably there is a way to relax this while still ensuring consistency, but
I do not see how. One idea is to store real path inside each entry. This
way we will be able to ensure that it is still at a valid location without
blocking parents, so concurrnecy will be restored. But we will have to
propagate strucutral changes to children. E.g. move of a folder with 100
items will lead to update of >100 cache entries. Not so good.

Any other ideas?

Vladimir.

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